feel like something from a nightmare., write this. Even just seeing it in text makes it feel like something from a
nightmare. She was, . It felt like all the best qualities one could imagine in a mother converged
in her. I don't know any, like national/ethnic conflicts, politics, religion, sexual orientation, and
gender identity, and any
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| 1 | Benq | 3676 |
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| 5 | tourist | 3408 |
| 6 | strapple | 3360 |
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You can use several words in query to find by all of them at the same time. In addition, if you are lucky search understands word forms and some synonyms. It supports search by title and author. Examples:
305— search for 305, most probably it will find blogs about the Round 305andrew stankevich contests— search for words "andrew", "stankevich" and "contests" at the same timeuser:mikemirzayanov title:testlib— search containing "testlib" in title by MikeMirzayanov"vk cup"— use quotes to find phrase as istitle:educational— search in title
Results
1.
likes
Last visit:
5 years ago
Registered:
5 years ago
2.
Please, read this Hello, Codeforces. Friends.
I apologize, but today I will be a bit chaotic and mix personal with public. It just happened that way, I'm sorry.
Today, my mother passed away. It's hard for me to write this. Even just seeing it in text makes it feel like something from a nightmare.
She was extraordinary. It's difficult for me to articulate it right now, but take my word for it. It felt like all the best qualities one could imagine in a mother converged in her. I don't know any other people so kind, sincere, non-judgmental, capable of love, listening, befriending, supporting, and taking joy in the world around them, and bringing joy to their loved ones. Mom, it's already hard without you, it feels empty.
She had glioblastoma. It's an aggressive brain cancer that affects only a few people out of 100,000. I believe, I hope, that science will soon be able to tackle this disease. But for now, this diagnosis is a death sentence. Maybe some of you are or will be associated with medic...
3.
Congratulations: Tourist has reached a rating of 4000! Hello,
As you may have already noticed, [user:tourist,2024-08-31] has reached the symbolic milestone of 4000 rating points. This is the first time in the history of competitive programming. I, personally, and the entire Codeforces team, congratulate Gennady [user:tourist,2024-08-31] Korotkevich on this incredible achievement!
Right now, we have assigned a new rank “Tourist” to the 4+ rating. This highlights the uniqueness of the moment!
However, in the future, I would like to change this name to something a bit more neutral (or leave it Tourist?). Would you help me with this?
I’d appreciate your cool suggestions in the comments!
Here are a few options from me. Please, vote:
- Epic Grandmaster
- Ultimate Grandmaster
- Godlike Grandmaster
- Tourist
- 4K Grandmaster
- Grandmaster 4K
- Legendary Grandmaster 4K
- Joke option: Ultra HD Grandmaster
- from [user:Tlatoani,2024-08-30]: Champ...
. This highlights the uniqueness of the moment! However, in the future, I would
like to change, However, in the future, I would like to change this name to something a bit
more neutral (or leave
4.
The Ultimate Topic List (with Resources, Problems and Templates) This post took $4$ years to make. And this is the most significant thing that I have ever shared in my whole life.
#### Story
Hi, I have been doing CP for like $4$ years and from the very beginning what I have been feeling is a need for a comprehensive topic list that will contain all sorts of topics from easy to advanced with corresponding tutorials, problem lists and templates so that I wouldn't have to look at different sites, from here to there. So what do you do when you think something is missing from the world? Yeah, you create that thing! So here I am, sharing the ultimate topic list that you will need in CP.
When I say that it took me $4$ years to make it, I genuinely mean it. I have been collecting them from the inception of my CP journey and yesterday I thought that it got its almost complete shape. You may not imagine the sheer excitement hidden under each of the characters of this post.
#### Payment
You can pay me just by upvoting this blog and by being a bet...
my whole life. #### Story Hi, I have been doing CP for like $4$ years and from
the very, #### Additional Comments I really wanted to post this blog before I die. Seems
like I managed to, #### Story Hi, I have been doing CP for like $4$ years and from the very
beginning what I have
5.
Retiring from competitive competitive programming Recently, I feel I've passed my peak as a competitive programmer, and ran out of sufficient energy/motivation to keep the top form and be prepared to win the next contest.
I think I've done everything I can do as a competitive programmer, and now it's time to end my career as a professional (mind) sport player.
It was a great experience.
I'd like to thank everyone involved in the community, especially:
- Competitors. It was always nice to compete against you all!
- Problem writers who gave great problems to the community.
I started my career as a competitive math-puzzle solver in 1999.
The first contest was a local one named "junior sansu (arithmetic) olympiad".
Since then, I enjoyed this sport a lot for 21 years.
In 2007, I started TopCoder, and it changed my life.
The problems were great, and every single match of SRM felt like a once-in-a-life event.
A lot of legends were competing there — [user:Petr,2020-12-31], [user:tomek,2020-12-31], [user:ACRush,2020-12...
and top competitors gather, and compete against each other for solving ad-hoc,
math-puzzle-like, match of SRM felt like a once-in-a-life event. A lot of legends were competing
there &mdash, ) sport player. It was a great experience. I'd like to thank everyone involved
in the community, It was a great experience. I'd like to thank everyone involved in the
community, especially:
6.
Codeforces Round #752 #### UPD: (28 April, 2022)
<spoiler summary="Donated Finally">
Details: [here](https://codeforces.me/blog/entry/96333?#comment-907470)
</spoiler>
আবার চলে এসেছি! (That's Bengali for "I am back! (in Terminator mode)")
I am super excited to invite you to participate in [contest:1603] and [contest:1604] which will be held on [contest_time:1604]. This round is rated for both divisions.
You will be given $6$ problems in each division and $2$ hours to solve them. All the problems are authored and prepared by me.
I would like to thank -
- [user:antontrygubO_o,2021-10-26] for his breathtaking(<strike>literally</strike>) coordination of the round. <i style="color:White;"> no funny text this time</i>
- [user:Alpha_Q,2021-10-26], [user:Anachor,2021-10-26] and [user:antontrygubO_o,2021-10-26] for putting up with my dumb ranting and helping with the problems.
- [user:Um_nik,2021-10-26], [user:gamegame,2021-10-26], [user:kefaa2,2021-10-26], [user:dorijanlendvaj,2021-10-26], [u...
**UPD:** Here is how the donation amount looks like:, Finally, I would like to dedicate this contest to me! I want to thank me for
believing in me, for, I would like to thank -
Announcement of Codeforces Round 752 (Div. 1)
Announcement of Codeforces Round 752 (Div. 2)
7.
Codeforces Round #712 omg hi!
I am pleased to invite you to participate in [contest:1503] and [contest:1504]! You will be given **6 problems** and **2 hours 15 minutes** to solve them. I'm happy to announce the theme of this round is déjà vu!
I would like to thank the following people:
- <a class="rated-user user-red" href="https://images.emojiterra.com/twitter/512px/1f921.png" title="The Entire Circus">antontrygubO_o</a> for accepting all of my problems <span style="color:white;">at gunpoint</span>
- <a class="rated-user user-orange" href="/profile/Nika_Tamliani" title="Useless Tester Nika_Tamliani">Nika_Tamliani</a>, <a class="rated-user user-violet" href="/profile/arvindr9" title="Useless Tester arvindr9">arvindr9</a>, <a class="rated-user user-red" href="/profile/dorijanlendvaj" title="Useful Tester dorijanlendvaj">dorijanlendvaj</a>, <a class="rated-user user-blue" href="/profile/I_Love_YrNameCouldBeHere" title="Useful Tester I_Love_YrNameCouldBeHere">I_Love_YrNameCouldBeHere</a>, <a class="r...
I would like to thank the following people:
Announcement of Codeforces Round 712 (Div. 1)
Announcement of Codeforces Round 712 (Div. 2)
8.
Codeforces Round #810 Editorial Sorry for the late editorial. May this editorial help you. If you have questions, feel free to ask.
[problem:1711A]
<spoiler summary="hint1.">
The minimal weight is at least $1$ since $1$ divides any integer (so $1$ divides $p_1$).
</spoiler>
<spoiler summary="solution">
Since $k+1$ does not divide $k$, a permutation with weight equal to $1$ is: $[n,1,2,\cdots,n-1]$.
</spoiler>
<spoiler summary="code">
~~~~~
#include <bits/stdc++.h>
using namespace std;
void work()
{
int n;
cin>>n;
cout<<n<<' ';
for (int i=1;i<n;i++)
cout<<i<<' ';
cout<<endl;
}
int main()
{
int casenum=1;
cin>>casenum;
for (int testcase=1;testcase<=casenum;testcase++)
work();
return 0;
}
~~~~~
</spoiler>
[problem:1711B]
<spoiler summary="hint1.">
See the party as a graph.
</spoiler>
<spoiler summary="hint2.">
Divide the vertices into two categories according to their degrees' parity.
</spoiler>
<spoiler summary="solution">
Let's consider ...
should be like:, ,r_1],\cdots,[l_{k+1},r_{k+1}](x\in [l_0,r_0],y\in [l_{k+1},r_{k+1}])$, you can
link edgeslike:, }$ and $col_{x,y-1},col_{x+1,y-1}$. Repeat such process, we will find the
boundary should belike
Tutorial of Codeforces Round 810 (Div. 1)
Tutorial of Codeforces Round 810 (Div. 2)
9.
Codeforces Global Round 9 <img src="/predownloaded/2e/e2/2ee2f0f95f05aae6ca6f47812dae2dc2aabf277b.png" style="width:200px; float:right; margin: 0 1em 1em 1em"/>
Hi!
On [contest_time:1375] we will host [contest:1375].
It is the third round of a 2020 series of [Codeforces Global Rounds](/blog/entry/65002). The rounds are open and rated for everybody.
The prizes for this round:
- 30 best participants get a t-shirt.
- 20 t-shirts are randomly distributed among those with ranks between 31 and 500, inclusive.
The prizes for the 6-round series in 2020:
- In each round top-100 participants get points according to the table.
- The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
- The best 20 participants over all series get sweatshirts and place certificates.
Thanks to XTX, which in 2020 supported the global rounds initiative!
The problems of this round were prepared by a team of authors: [user:adamant,2020-07-01], [user:anto...
even if I don't like the problems. Please participate!_ - _As a tester, I
tested the round._ - _As, would like to thank the following people:
Announcement of Codeforces Global Round 9
10.
Hello 2024 Hello Codeforces,
We are very glad to invite you to participate in [contest:1919], which will start on [contest_time:1919]. You will be given **8 problems** and **2.5 hours** to solve them. One of the problems will be divided into two subtasks. The round will be rated for everyone. There will be at most 2024 interactive problems, so please read the [guide for interactive problems](https://codeforces.me/blog/entry/45307) before the contest.
All the problems are written and prepared by me.
<spoiler summary="Spoiler">
The first tester tested on 10 October **2022**.
</spoiler>
We would like to give our sincere thanks to:
- [user:errorgorn,2024-01-02] for his wonderful coordination!
- [user:Alexdat2000,2024-01-02] for translating problem statements.
- [user:dario2994,2024-01-02] for coming up with the solution to one of the problems.
- [user:conqueror_of_tourist,2024-01-02], [user:iLoveIOI,2024-01-02], [user:Um_nik,2024-01-02], [user:oolimry,2024-01-02], [user:thenymphs...
would like to give our sincere thanks to: - [user:errorgorn,2024-01-02] for
his wonderful, We would like to give our sincere thanks to:
Announcement of Hello 2024
11.
Rule Restricting the Use of AI [revision 2024-09-14] It seems that neural networks are working technological wonders. Not long ago, they struggled with even the simplest tasks in our competitions, but now they are reaching new heights that cannot be overlooked.
There is reason to believe that progress will continue, and we can expect further advancements in neural networks within the realm of programming competitions.
For this reason, we are explicitly limiting the use of AI-based systems (such as various models like GPT, Gemini, Gemma, Llama, Claude, and others) for solving programming problems.
However, we recognize that AI can be a valuable tool for learning and coding assistance, so we aim to establish clear boundaries for its use.
**Application of this Rule:**<br/>This rule applies strictly to in-contest participation. This includes both registered contestants and out-of-competition participants in any rated Codeforces round. If a round is unrated and this is explicitly stated in the round announcement or rules, the rul...
explicitly limiting the use of AI-based systems (such as various models like
GPT, Gemini, Gemma, ., after receiving a rejected verdict like "Runtime error on test 1" or
similar, you are prohibited from, like GPT, Gemini, Gemma, Llama, Claude, and others) for solving programming
problems.
12.
New Year's Eve Update: Random Problems for Mashups Hello, Codeforces!
Only a couple of days remain until 2025. To wrap up the year, I'm bringing an update — I hope you'll find it useful.
Now, you can automatically find and add random problems for your mashup based on a range of criteria and restrictions.
The problems are sourced from the archive, and unusual problems are automatically excluded.
So, click the "Add random problems" link under the problem list and proceed to the addition form.
<center>
<img style="zoom:75%;" src="/predownloaded/b9/25/b925e4d00215ff2e408cc81c2629936df72dce38.png"/>
</center>
Here's what you can do:
* Exclude problems that registered mashup participants have solved/attempted/authored (a problem will be excluded if at least one registered participant is familiar with it),
* If the mashup belongs to a group (e.g., your training group), exclude all problems from other contests in that group,
* Exclude problems with subtasks — such problems are often interconnected or referenc...
you like and finally add them to your mashup., . In this list, you can mark the problems you like and finally add them to your
mashup. Note that
13.
Codeforces Round #639 Hello, Codeforces!
I'm very glad to invite you to [contest:1344] and [contest:1345]. This contest will take place on [contest_time:1344]. In both divisions, you will have **2 hours 15 minutes** to solve **6 problems**. The score distribution will be announced closer to the start of the contest.
Of course, this round would not be possible without the help of many people, who I'd like to thank:
- [user:isaf27,2020-05-01], for coordinating this round, and helping with solution ideas.
- My ICPC teammates [user:Kuroni,2020-05-01] and [user:strongoier,2020-05-01], and coach [user:cn355,2020-05-01], for encouraging me to pursue competitive programming. In particular, I'd like to thank [user:Kuroni,2020-05-01] for being so spectacularly bad that a barely-orange user like myself actually has a place on our team.
- [user:300iq,2020-05-01], [user:antontrygubO_o,2020-05-01], [user:Ari,2020-05-01], [user:ffao,2020-05-01], [user:fpc_coder,2020-05-01], [user:gamegame,2020-05-01], [user:Kur...
-01], for encouraging me to pursue competitive programming. In particular, I'd
like to thank, . Of course, this round would not be possible without the help of many people,
who I'dlike to, Of course, this round would not be possible without the help of many people,
who I'dlike to thank:
Announcement of Codeforces Round 639 (Div. 1)
Announcement of Codeforces Round 639 (Div. 2)
14.
Codeforces Round #735 (Div. 2) কি অবস্থা মামা? (That's Bengali for "What's up dude?")
I am super excited to invite you to participate in [contest:1554] which will be held on [contest_time:1554]. This round is rated for future LGM participants whose current rating is $\le 2099$.
You will be given $5$ problems and $2$ hours to solve them. All the problems are authored and prepared by me.
I would like to thank -
- [user:antontrygubO_o,2021-07-20] for his awe-inspiring coordination of the round <i style="color:White;"> and rejecting my problems, and thus hurting my feelings.</i>
- [user:Um_nik,2021-07-20], [user:_Ash__,2021-07-25], [user:Anachor,2021-07-20], [user:Arg_007,2021-07-20], [user:Alpha_Q,2021-07-20], [user:DAleksa,2021-07-20], [user:steinum,2021-07-20], [user:tasdid,2021-07-20], [user:border,2021-07-20], [user:mk_Shahriar,2021-07-20], [user:t17,2021-07-20], [user:imAnik,2021-07-20], [user:Alfeh,2021-07-20], [user:Aritra741,2021-07-20], [user:FrozenBlood,2021-07-20], [user:SajidZakaria,2021-07-20]...
to solve them. All the problems are authored and prepared by me. I would like
to thank, I would like to dedicate this contest to the fallen king, my father, who died
two months ago. May, I would like to thank -
Announcement of Codeforces Round 735 (Div. 2)
15.
Codeforces celebrates its 15th anniversary! <img src="https://assets.codeforces.com/files/codeforces15.jpg" style="width:250px; float:right; margin: 0 1em 1em 1em;" />
Hi, Codeforces!
Around this time, 15 years ago, I was deploying the first version of Codeforces on a server, registering my account, and writing the first post. 15 years have passed. Well, happy anniversary to us!
For me, this is the most exciting adventure — full of ups and downs, hard work, and moments of joy. What's amazing is that it was never a lonely journey. I've always had caring and reliable friends by my side! That includes the Codeforces team, coordinators and problem writers, our partners in various events, all community members, and, of course, our sponsors — steadfast supporters and helpers.
Look at what we've accomplished together! Here are some of the key metrics over the years. There seems to be plenty to celebrate!
<center>
<img src="https://assets.codeforces.com/files/15/01-submissions.png" class="resource" style="width:300px;"/>...
**News #2.** In this context, I'd like to announce increased payouts for
problem authors and, -groups.png" class="resource" style="width:300px;"/> On a day like this, of
course, I want to, On a day like this, of course, I want to share some good news. And we have
some! We have fantastic
16.
Rounding 2025 Hi Codeforces!
2025 comes to an end. However square the year was, we held a number of rounds: 29 Div. 1 and 31 Div. 2-only contests, to be precise! Additionally, 18 Div. 3, 4 Div. 4, and 13 Educational rounds made the schedule even more congested.
<img src="/predownloaded/c8/54/c854d6a423b830263449cca6ff841c8e4e02e024.png" style="width:1400px; float:center; margin: 1em 1em 1em 1em"/>
December is not only the <strike>perfect</strike> [only](/blog/entry/149140?#comment-1332941) time to wrap the year, but also to celebrate and thank all the people who made this year possible. That's why I want to highlight with this post the people whose efforts shaped Codeforces contests.
I'd like to say a huge thank you to every active community member who created and/or prepared problems for Codeforces rounds! Without your ideas and creativity, it would be impossible to hold so fun and interesting competitions. There are so many of you from every corner of the world, so that it's impossible...
I want to highlight with this post the people whose efforts shaped Codeforces
contests. I'dlike, And last but not the least, I'd like to thank almost 800 testers who got to
know the problems, I'd like to say a huge thank you to every active community member who created
and/or prepared, Next, I'd like to extend my deepest gratitude to our coordinators who make sure
rounds are the best
17.
I don't want this to get normalized I don't think comments like "you use X in model solution, author, are you an idiot?", "f**k you and your problems, they are shit" and such should be normal on Codeforces, especially when written by someone high-rated and/or respected in the community. I also don't think this kind of behavior should be normal towards regular users.
I know that sometimes contests can be frustrating. Sometimes it is the participant's fault, sometimes it is the problemsetter's fault, sometimes it just happens without anyone being guilty of that. And I understand that in some cases, the criticism the author receives is fair and well-deserved.
But there is a fine line between criticism, saying that you didn't like the problems, and hurling insults against the author. The former two are acceptable (and sometimes even needed, because authors have to improve and learn from their mistakes); the latter one, in my opinion, should not be acceptable.
It's not like we can ever get rid of offense and hatred ...
acceptable, and they are allowed to do it as well. That's why cases like this
one are especially bad., doubt you will like the results of that., have made it seem like you're the culprit, and I didn't understand the
consequences of my blog, **UPD:** It looks like the focus of the comment section has shifted towards
discussing how to give, But there is a fine line between criticism, saying that you didn't like the
problems, and hurling, I don't think comments like "you use X in model solution, author, are you an
idiot?", "f**k you and, It's not like we can ever get rid of offense and hatred completely; there will
always be people who
18.
I Love CF I miss the old CF, Anton rounds cold CF
Textbook sized blogs CF, no cheater blogs CF
I hate the new CF, the captcha hell CF
No Div 1s scheduled CF, just endless Div 4s CF
I miss the sweet CF, debug till sleep CF
I gotta say pre-GPT I’d like to meet CF
See we invented CF, it wasn't any CF
And now I look and look around just AI cheaters CF
I used to love CF, I used to love CF
I even made the rounds, I thought I was CF
What if CF made a blog about CF
Called “I Miss the Old CF," man that would be so CF
That’s all it was CF, we still love CF
And I love you like CF loves CF
CF, debug till sleep CF I gotta say pre-GPT I’d like to meet CF See we
invented CF, it wasn't, I miss the sweet CF, debug till sleep CF I gotta say pre-GPT I’d like to meet
CF See we, That’s all it was CF, we still love CF And I love you like CF loves CF
19.
My opinion on how to practice competitive programming I'm not very often replying to questions about how to practice, but as I'm getting enough of them (and I've just seen another blog about practicing) let me tell you about it.
Of course, every time when you ask someone good about how to practice, he/she will reply to you to "solve a lot of problems" and that's true, there's no other way. Anyway, I've thought about it and actually I'm able to tell a bit more. I know some people, I've seen many people practicing (including me) and I have an opinion.
Many people practice in some organized way. High schools organize IOI/OI training contests every Saturday, universities organize ICPC training contests once a week, people try by their own to solve three problems each day, websites host rounds and so on. Here's a secret: it's a sh*t. Yep, that's true. If you want to be really good and to make it happen you compete in a training once a week, you do it only to be able to make excuses "but I'm training so hard" when you see no progress.
...
some games? Sure, if you don't feel like practicing, then don't force
yourself. If you really want to, like practicing, then don't force yourself. If you really want to practice and
become strong, you'll
20.
Streams on Codeforces Hello everyone!
Today Codeforces got support for streams! Thanks [user:tourist,2020-10-21] for the great idea!
There are two popular platforms supported: Twitch and YouTube. The opportunity to add streams to Codeforces is open to red participants and by individual invitations for now. We possibly open this opportunity to more users later.
The idea is that streamers will be able to announce their streams to the Codeforces audience. To do this, you should add the stream to the site using a special section in your profile. Please, note that when adding a Twitch stream, you just need to specify the URL of your channel, and when adding a YouTube broadcast, its unique short link (URL like youtu.be/...)
Shortly before the start of the stream, a notification about the upcoming stream will appear in the sidebar on the right, which will be shown to all site visitors.
For example, right now there is a [user:Errichto,2020-10-21]'s stream announcement in the sidebar.
). The statement goes as follows:
You are given $n$ words ($2 \leq n \leq 50\,000$), every of length exactly $5$ characters. Each character can be a lowercase letter, an uppercase letter, a digit, a comma... basically, it can be any character with ASCII code between $48$ and $122$ (let's say that $k$ is the number of possible characters). A task is to find all pairs of indexes of words which are $\textbf{really different}$. Two words are $\textbf{really different}$ if they differ at all $5$ corres...
will go well. Total complexity is something like $O(n \cdot 600 \cdot
log(n))$, but we can get, Total complexity is something like $O(n \cdot 600 \cdot log(n))$, but we can
get rid of the $log
22.
Goodbye 2020. Hello 2021! <img src="/predownloaded/f4/1f/f41ff0beb745fb2f84a55d9f2cc451273efc6267.jpg" style="width:300px; float:right; margin: 0 1em 1em 1em"/>
Hello Codeforces!
This unusual and difficult 2020 has come to an end! What will the new year be like?
I wish all of us that we have the opportunity to meet in person so that the contests take place not only online.
I ask you to think of something important and good. Let this wish come true!
We have had a large number of rounds this year. I hope we have pleased you with interesting problems.
Thanks to the round coordinators and writers! You make our community richer. Please make us happy in 2021!
I wish the participants victories over yourself (and over problems!) and development of your skills, interest and curiosity!
Hello, 2021!
And we have traditional gifts!
#### <img src="https://assets.codeforces.com/images/wand.png"/> Change Handle Feature
Hurry! Only until the 10th of January, you can change your handle (<b>but only onc...
to an end! What will the new year be like? I wish all of us that we have the
opportunity to meet, with it. Especially, I do not like requests like "this is my second account, I
took an interesting, This unusual and difficult 2020 has come to an end! What will the new year be
like? I wish all of
23.
Codeforces Round #648 (Div. 2) Hi everyone!
I would like to invite you to my fifth Codeforces Round, that I set with my friends [user:FastestFinger,2020-06-05], [user:Vivek1998299,2020-06-05] and [user:ridbit10,2020-06-05].
We are excited to bring another contest within a week :D
With that said, I bring to your attention our new [contest:1365] that will take place on [contest_time:1365]. If your rating is less than **2100**, this round will be rated for you; otherwise, you can participate out of competition.
I would really like to thank:
- [user:FastestFinger,2020-06-05], [user:Vivek1998299,2020-06-05] and [user:ridbit10,2020-06-05] for helping with preparing problems and giving new ideas.
- [user:300iq,2020-06-05] for coordinating our round.
- [user:Um_nik,2020-06-06], [user:antontrygubO_o,2020-06-05], [user:ksun48,2020-06-05], [user:Jeel_Vaishnav,2020-06-05], [user:aryanc403,2020-06-05], [user:FlowerOfSorrow,2020-06-05], [user:KonaeAkira,2020-06-05], [user:_Aaryan_,2020-06-05], [user:Ari,2020-06-...
Hi everyone! I would like to invite you to my fifth Codeforces Round, that I
set with my friends, I would like to invite you to my fifth Codeforces Round, that I set with my
friends, I would really like to thank:
Announcement of Codeforces Round 648 (Div. 2)
24.
Codeforces Round #646 (Div. 2) Hi everyone!
I would like to invite you to my fourth Codeforces Round, which I have made with my friends [user:FastestFinger,2020-05-29] and [user:TheOneYouWant,2020-05-29]. In terms of problems, it is my favorite among all my rounds.
With that said, I bring to your attention our new [contest:1363] that will take place on [contest_time:1363]. If your rating is less than **2100**, this round will be rated for you; otherwise, you can participate out of competition.
I would really like to thank:
- [user:FastestFinger,2020-05-29] and [user:TheOneYouWant,2020-05-29] for helping with preparing problems and giving new ideas.
- [user:antontrygubO_o,2020-05-29] for coordinating our round
- [user:300iq,2020-05-29], [user:Jeel_Vaishnav,2020-05-29], [user:Ari,2020-05-29], [user:aryanc403,2020-05-29], [user:mahmoudbadawy,2020-05-29], [user:Vivek1998299,2020-05-29], [user:Uzumaki_Narutoo,2020-05-30], [user:Bakry_,2020-05-31], [user:smartnj,2020-05-30] and [user:Nikhil_Medam,2020-05-29]...
Hi everyone! I would like to invite you to my fourth Codeforces Round, which I
have made with my, I would like to invite you to my fourth Codeforces Round, which I have made
with my friends, I would really like to thank:
Announcement of Codeforces Round 646 (Div. 2)
25.
Good Bye 2025 <div style="overflow:hidden;">
<img src="/predownloaded/a2/4f/a24f8db165991fb983fc766d223782cbf2785f7f.png"
style="float:right; width:200px; height:auto; margin-left:50px; margin-right:50px; border-radius:0px;" />
<h2>Merry Christmas, Codeforces! 🎄 🎄 🎄</h2>
<p>
We are glad to invite you to take part in [contest:2178], which will start on [contest_time:2178]. You will be
given <b>9 problems</b> and <b>3 hours</b> to solve them. <b>At least one problem will be interactive</b>, so please make sure to read the <a href="https://codeforces.me/blog/entry/45307">guide for interactive problems</a> before the contest. This round is open and rated for everyone.
</p>
<p>
Problems are authored by [user:conqueror_of_tourist,2025-12-25], [user:twosquares,2025-12-25], and [user:YuukiS,2025-12-25].
</p>
<p>
We would like to thank:
</p>
</div>
<div style="clear:both;"></div>
- [user:Error_Yuan,2025-12-...
We would like to thank:
, > We would like to thank:
- [user:Error_Yuan,2025-12
Announcement of Good Bye 2025
26.
Goodbye A2 Online Judge :( I'm sadly announcing that [a2oj.com](https://a2oj.com/) ([A2 Online Judge](https://www.facebook.com/A2OnlineJudge/)) will be retiring by end of 2019, I'll shut it down at midnight on New Year's Eve. It was something I took care of for about 10 years.
I'm really sorry, but I prefer not to explain my reasons, so please don't ask me why I'm doing this, it was hard enough for me to make this decision, and also please don't try to convince me to change my mind.
I'd like to say thank you to everyone who used it, and please accept my sincere apologies for shutting it down.
If you have something good to say about A2OJ (how you used it or how it helped you in any way), I'd love to read that, please send it to the following email (or in a comment here): [email protected]
Also please feel free to take a copy of any data you can access, and if you need some data in a specific format, please send your request to the following email: [email protected] (if the data isn't public, make sure to send f...
decision, and also please don't try to convince me to change my mind. I'd like
to say thank you to, I'd like to say thank you to everyone who used it, and please accept my sincere
apologies for
27.
Goodbye 2021. Hello 2022! <img src="/predownloaded/f4/1f/f41ff0beb745fb2f84a55d9f2cc451273efc6267.jpg" style="width:300px; float:right; margin: 0 1em 1em 1em"/>
Hello Codeforces!
Oh yes, 2021 ends! I recently realized that 2022 is pronounced like "2020 too". Not that. I want a completely different year!
Anyway, we did a nice job this year. I hope we have pleased you with interesting problems.
Thanks to the round coordinators and writers! You make our community richer. Please make us happy in 2022!
I wish you all interesting tasks and brilliant solutions! I wish you error-free code and wish us only rated rounds :-)
Hello, 2022!
And we have traditional gifts!
#### <img src="https://assets.codeforces.com/images/wand.png"/> Change Handle Feature
Hurry! Only until the 10th of January, you can change your handle (<b>but only once</b>)! Note that it will be possible to roll back the changes or change the handle again only _after a year_. Be careful what you wish for.
You can change your han...
that 2022 is pronounced like "2020 too". Not that. I want a completely
different year! Anyway, we, with it. Especially, I do not like requests like "this is my second account, I
took an interesting, Oh yes, 2021 ends! I recently realized that 2022 is pronounced like "2020 too".
Not that. I want a
28.
Codeforces: Updates in rating and rounds <img src="/predownloaded/21/a3/21a33b497ee8a34a678befc3f79745eb69a9cf07.png" style="float:right; margin: 0 1em 1em 1em;"/>
Hello, Codeforces!
It's time for news. I hope you will enjoy!
* We reduce the rating bound for the orange color. Thus, now the orange rating (and the _Master_ title) will start from 2100 rating units.
* The lower bound of the rating for _legendary grandmasters_ is increased to 3000 rating units. Thus, right now in the community, 14 participants meet this requirement. Real legends!
* For rounds like "Div. 2 Only" the upper bound of the rating for official participation is increased. Now everyone whose rating is less than 2100 (i.e., before _masters_) will be able to register for such rounds to participate officially, their rating will be updated based on the results of the round. We do not plan to increase the complexity of Div. 2 rounds. Practice shows that the problems of such rounds are interesting and useful for participants "below the orange".
...
requirement. Real legends! * For rounds like "Div. 2 Only" the upper bound of
the rating for official, * For rounds like "Div. 2 Only" the upper bound of the rating for official
participation is
29.
Red Isn't Impossible _I waited for more than a year to write this blog._
I started CP in July 2017, I always dreamed about reaching red, and it finally happened. I didn't become red like talented contestants in 1-2 years, but it took me more than that.
The start of my CP journey was average (you can see my graph); It took me more than six months to reach pupil, then another six months to get specialist, and it took me more time to achieve higher ranks. I didn't have any mentor or coach. So, I asked many stupid questions, which you can find if you read my old blogs. One of these questions is, "What's the importance of Dynamic Programming?" (I drafted this blog xD), But of course, I don't motivate people to ask these types of questions as they usually can be answered by just searching.
I failed many times during my journey in CP, especially in important competitions (IOI21 and IOI22). I stopped CP many times because I was frustrated and believed I could never improve. For example, I stopped when I w...
about reaching red, and it finally happened. I didn't become red like talented
contestants in 1-2, become red like talented contestants in 1-2 years, but it took me more than
that.
30.
Good Bye 2019 Hello again, Codeforces!
We are glad to invite you to <s>Mathforces</s> <s>Thinkforces</s> [Good Bye 2019](https://codeforces.me/contests/1270), which will take place on [contest_time:1270].
Some information about the round:
<ul>
<li>Rated for all participants!</li>
<li>3 hours!</li>
<li> <strong> No subtasks! </strong> </li>
<li>There will be an <strong> interactive problem </strong> in this round. You can read the guide for interactive problems [here](https://codeforces.me/blog/entry/45307)</li>
<li> Editorial will be published right after the system testing </li>
</ul>
All problems in this round were prepared by us, [user:antontrygubO_o,2019-12-28] and [user:kefaa2,2019-12-28]. We worked on this round for a long time and tried to make all the problems very interesting. We hope that you will enjoy the round!
We would like to thank:
<ul>
<li> [user:arsijo,2019-12-24] for coordination of the round </li>
<li> [user:GandalfTheGrey,2019-12-24] fo...
UPD1: Using opportunity, we would like to advertise [the match](https, very interesting. We hope that you will enjoy the round! We would like to
thank: , We would like to thank:
Announcement of Good Bye 2019
31.
Goodbye 74TrAkToR + Problem A~E: The problems themselves are good and typical A~E. Indeed I like problem D. The pretest for A is a bit weak, and it's not a big problem yet.
+ Problem F: The problem has an origin. :) See the link: [click](https://www.luogu.com.cn/problem/P9394). The problem in contest is only a weakened version of the one in Luogu.
+ Problem G: It's said that our great coordinator had not proved the time complexity of the intended solution is correct :) If the authors did, please share it in the editorial. (although this problem is completely beyond my ability :) )
+ Problem H: Oh dear [user:74TrAkToR,2023-12-30], could you please OEIS the sequence before you use the "several-integer-input" problem in
rounds next time? Anyone who copied the first example and opened [https://oeis.org/A286331](https://oeis.org/A286331) could quickly get the formula. And the problem itself is not so hard imo. Anyway, it should not be used in contest, especially for the last problem.
For me pers...
problems occurred in the round, but it feels like Div1A=B=C=D for me :( And
actually the performance, + Problem A~E: The problems themselves are good and typical A~E. Indeed I like
problem D. The
32.
Codeforces: Results of 2023 [Annual Report] Hello, Codeforces.
Let's recall 2023, shall we?
In this post, I want to summarize this year. You know, a lot of terrible things happen in the world, and personally, this year was the hardest and most sorrowful for me. But today, we'll remember something else. We'll remember what this year was like for Codeforces.
Recently, Codeforces turned 14 years old. Not bad, right? And for many years now, when asked "What is Codeforces?" the first thing I say is — it's a community!
[cut]<br/>
#### Personal Thanks!
On behalf of the community (and myself), I say thank you to everyone who tries, helps, and makes efforts to keep the community alive.
Thank you to the developers! Although I continue to engage in development and other technical activities, I'm glad that these guys made an effort and improved the platform: [user:geranazavr555,2024-02-04], [user:unreal.eugene,2024-02-04], [user:vbandurin,2024-02-04], [user:ChurakovaAlexandra,2024-02-04], [user:medvezhonokok,2024-02-04]...
something else. We'll remember what this year was like for Codeforces.
33.
Seems like a notorious coincidence #### _1 minute before contest starts_
Alright, this is it. The round is starting soon. I hope everything goes well!
#### _9 minutes after contest starts_
...What? Someone solved H already? Uh oh.
Okay, maybe they are just really good at XOR problems.
#### _40 minutes after contest starts_
Oh! An LGM solved it. Let's look at their submission...
~~~~~
// https://yukicoder.me/problems/no/2990
~~~~~
Oh. Oh no. Oh no no no no no.
---
Hi, so to me seems like a notorious coincidence.
I was the author of problem H, and unfortunately, it exactly coincides with [No.2990 Interval XOR](https://yukicoder.me/problems/no/2990) on yukicoder.
Just to be **absolutely clear**, I had the idea for H long before that:
- I thought of problem H in January 2024.
- The problem was proposed in February 2024.
- The problem was approved in October 2024.
- The problem on yukicoder was released in December 2024.
Here are some obligatory evidence screenshots:
<spoile...
Seems like a notorious coincidence, /problems/no/2990 ~~~~~ Oh. Oh no. Oh no no no no no. --- Hi, so to me seems
like a, Either way, sorry that this happened. I remember when I spent like 5 hours
staying up all night, Hi, so to me seems like a notorious coincidence., Make the round unrated? It seems like a reasonable option, but the fact that it
isn't trivial to
34.
Please try to be more positive in your feedback on the rounds. Hi, a few disclaimers first.
1. I work as a coordinator on CF, but this blog is solely my initiative (I didn't tell anyone from CF about it).
2. I was a few times in the author role, so I think that I have the right to speak on behalf of both authors and coordinators. Obviously, it doesn't mean that all authors/coordinators think the same, but (based on discussion with some people) I think that my opinion is not unpopular.
Main point of this blog — I feel like our community is becoming much more aggressive towards any things (some of which I can't even call issues) that they don't like during the round. I don't think it's a good direction, and I think it can decrease the motivation of authors & coordinators to do more rounds.
Now I will explain why I think so.
Obviously, you might guess that this blog was inspired by yesterday's round, but also a few recent ones too (for instance, CF 1069, CF 1058).
The first thing that any person will see on the home page now is that...
that my opinion is not unpopular. Main point of this blog — I feel like our
community is becoming, where you write not only about the things you didn't like, but also the things
that you liked there, Main point of this blog — I feel like our community is becoming much more
aggressive towards any
35.
New: Diagnostics of Solutions in C++ <img src = "/predownloaded/b5/d2/b5d246ffc32dfe82c3cbac77d1898c5fcb62fa57.jpg" style = "float: right; margin: 0 1em 1em 1em;"/>
Hello!
I am pleased to report that two rounds of Codeforces have gone quite well in terms of the work of Codeforces, I am very glad about it. These <s>days</s> nights I spent in a profiler, fixing the code, researching the settings of MariaDB.
In addition, I managed to allocate several hours on Sunday (to be honest, until Monday morning) to finish the long-planned innovation.
Meet, diagnostics of solutions in C++!
Many Codeforces visitors are already tired of the questions of less experienced participants: "Why does my solution not work on some test on the Codeforces servers, if I locally launch it and it works correctly? You have the wrong compiler/servers!" In 99% of cases this is an example of <a href ="https://en.wikipedia.org/wiki/Undefined_behavior">undefined behavior</a> in a program. In other words, the program contains mistakes that, du...
some cases will prevent a question like "Why does not work ???", indicating
the error or its, Now automatic diagnostics in some cases will prevent a question like "Why does
not work, Thus, the diagnostics will sometimes help you to find a mistake like "out of
bounds error
36.
Giving negative feedback to contests in comments //This is a short post motivated by the comment section of the last Div2 round (and of several rounds before)
Every problemsetter tries to make his contest as good as possible, and is upset when something goes wrong. Most problemsetters don't set contests for money, the payments are not worth the time and efforts spent on coming up with problems, preparing them, and hosting the contest. The most rewarding thing in this process in not the money, it's to see contestants enjoying your problems.
That's why feedback is so important. If the comment section is filled with comments praising the contest, setter will most likely want to come back with another contest, and if it's filled with hate and offenses, it's possible that setter will just give up on problemsetting and won't want to host another round.
This means two things.
First, if you liked the round, or some particular problem, why not to write a comment about it? I am sure that most setters would really appreciate it, a...
problems like the one you liked. Positive comments from high rated
contestants, or from experienced, they didn't like about the contest, so that setter can avoid such mistakes in
the future., , and feel more motivated to come up with problems like the one you liked.
Positive comments from high
37.
Corruption, Cheating at ICPC Yinchuan This blog is about Corruption and Cheating at ICPC YinchuanRegional. ICPC Yinchuan Regional is one of the ICPC regional contests for students competing in the geographic area of Asia East Continent.
I. Background: General Info
------------------
Ningxia Institute of Science of Technology, or NXIST is a higher education institute in China. Since 2019, NXIST has held two regional contests. The and both were considered by many participants as jokes, yet until today participants are unaware of the NXIST's extent of evil.
Zhihu is a Chinese website similar to Quora, and I will refer to this website for information.
II. Background: Before 2021
------------------
Regional competitions in China consists of two stages: The online selection contest and the onsite regional contest. As there are many contestants in China, the online contest is considered serious and rules are usually strictly enforced, with entire universities where some participant cheats disqualified from attend...
I would like to post this on my main account, but then I am not looking for
trouble directed at me.
38.
Codeforces Round 1134 (Div. 4) Hello Codeforces!
I am glad to invite you to [contest:2290], which will take place on
[contest_time:2290]. After quite a long break, I have taken up problem setting
again — this time working together with AI agents. For me, this is an
experiment in getting back into problem writing, and we will see together how
successful AI turns out to be as a partner in this work. I am looking forward
to seeing both Div. 4 participants competing officially and everyone else
joining out of competition!
<div style="height:1em;"></div>
<spoiler summary="Why I decided to author this round (a bit of lyrics and rambling)">
Yes, I am dictating these words into my phone right now, to put them into shape
for you a bit later.
We live in an amazing time when so many things are changing. I have been
programming for more than 25 years now, and throughout almost all of this time
I wrote code every day and solved problems every day. At first, these were
mostly programming contest problems — ...
, like a float on the water, for many years now., After solving literally just a few problems, almost like magic, new problem
ideas and little
39.
AtCoder Library Recently, the number of algorithms and data structures we use in competitive programming are rapidly growing.
It's a nice thing: by using more algorithms, the variety of possible problems gets wider, and we can enjoy more problems.
On the other hand, before reaching adhoc, thinking-oriented part of this competition, we have to spend more and more time to learn algorithms.
Sometimes a problem asks matching on general graphs; you have to find a paper describing it, read it, and implement its really complicated algorithm. Or sometimes you have to spend time tuning your library by a constant factor. Or sometimes you use multiple pre-written codes together, the variable names collide, and get annoyed.
Until now, I basically rejected all problems that require pre-written codes of complicated algorithms because I don't like these things. For example, we never used segment trees with lazy propagation in our contests.
However this way we can't use otherwise interesting problems and it ...
algorithms because I don't like these things. For example, we never used
segment trees with lazy propagation, complicated algorithms because I don't like these things. For example, we
never used segment trees with, have. Should we include more advanced algorithms (like matchings on general
graphs)? Our main, tasks that are irrelevant to the library, so don't try to think problems like
"ok, maybe this task
40.
Self-deception: maybe why you're still grey after practicing every day I generally don't like to give much advice on "how to practice", but recently I have been thinking a lot about this and I realized that there is something that I believe affects a lot of people (though it is admittedly hard to tell) that barely gets talked about: self-deception.
Self-deception is a devious thing. It can be very hard to detect, yet can be fatal to improvement.
This blog is mainly directed to greys, greens and cyans that can't get out of these categories. Most examples are given with that in mind. That being said, I believe others have something to think about as well.
This is my first blog on such "psychological issues" and I don't know if I'll ever make another one. It's certainly a difficult topic to write about because for most of the blog, the message is "your practice is not as good as you think it is" or even "you aren't as good as you think you are" which can of course be a hard thing to hear. Thus, I have to somehow be very gentle while getting the poin...
let you deceive yourself. USACO training solved this problem like 25 years ago
(I'm guessing based, some statements in editorials that kind of look like techniques you're
supposed to learn., started to really like doing virtual contests. I enjoyed the process of doing
them and started doing them, I generally don't like to give much advice on "how to practice", but recently I
have been thinking, To start, I'll give a personal story as to not seem like I'm talking from a
high horse. Self, like I slogged through them — just kinda did them, with overall less interest
in solving the
41.
Codeforces Round #651 (Div. 2) Hi everyone!
I would like to invite you to another one of our rounds, that I set with my friends [user:FastestFinger,2020-06-05], [user:smartnj,2020-06-19], [user:the_hyp0cr1t3,2020-06-19] and [user:ridbit10,2020-06-05].
The round [contest:1370] will take place on [contest_time:1370]. If your rating is less than **2100**, this round will be rated for you; otherwise, you can participate out of competition.
I would really like to thank my co-setters and:
- [user:isaf27,2020-06-05] for coordinating our round.
- [user:cerberus97,2020-06-19], [user:Devil,2020-06-19], [user:Taran_1407,2020-06-19], [user:Alireza_Keshavarz,2020-06-19], [user:Vivek1998299,2020-06-19], [user:KonaeAkira,2020-06-19], [user:demoralizer,2020-06-19], [user:Dragnoid99,2020-06-19], [user:agtxdy,2020-06-19], [user:Rajdeep,2020-06-19], [user:aryanc403,2020-06-19], [user:lavish315,2020-06-19], [user:DeadlyCritic,2020-06-19], [user:Utkarsh.25dec,2020-06-19], [user:AmShZ,2020-06-19], [user:_Aaryan_,2020-06-19]...
Hi everyone! I would like to invite you to another one of our rounds, that I
set with my friends, I would like to invite you to another one of our rounds, that I set with my
friends, I would really like to thank my co-setters and:
Announcement of Codeforces Round 651 (Div. 2)
42.
CodeTON Round 5 (Div. 1 + Div. 2, Rated, Prizes!) **Note the unusual start time of the round.**
Hello, Codeforces!
Now that Gaokao is over, we are very glad to invite you to participate in [contest:1842], which will start at [contest_time:1842]. You will be given **9 problems** and **3 hours** to solve them. The round will be rated for everyone.
All problems are written and prepared by [user:Gary2005,2023-06-21], [user:Asuka,2023-06-21], [user:Crying,2023-06-21], [user:sjcsjcsjc,2023-06-21], [user:MonkeyKing,2023-06-21], [user:DerekFeng,2023-06-21], [user:KbltQaQ,2023-06-21], [user:ShmilyTY,2023-06-21] and me.
Statements and editorials will be available in Chinese (Simplified) after the contest.
We would like to give our sincere thanks to:
- [user:errorgorn,2023-06-21] for his wonderful coordination!
- [user:Alexdat2000,2023-06-21] for translating problem statements.
- [user:gyh20,2023-06-21], [user:wangziji,2023-06-21], [user:flowerletter,2023-06-21], [user:themoon,2023-06-21], [user:qiuzx,2023-06-21], [user:Gezh...
editorials will be available in Chinese (Simplified) after the contest. We
wouldlike to give our, polygon platform. - Lastly, we would like to express our gratitude to **you**
for participating in the, We would like to give our sincere thanks to:
Announcement of CodeTON Round 5 (Div. 1 + Div. 2, Rated, Prizes!)
43.
Codeforces Round 1121 (Div. 2) <div class="problem-statement" style="border:0 !important; padding:0 !important; margin:0.5em !important; box-shadow:none !important;">
<div class="header">
<div class="title">1121. Another Round</div>
<div class="time-limit"><div class="property-title">time limit per participant</div> 2 hours</div>
<div class="memory-limit"><div class="property-title">memory limit per participant</div> 1 brain</div>
</div>
<div class="legend">
<p>
<i>This is <s>an interactive problem</s> a contest with an <b>unusual start time</b>.</i>
</p>
<p>
You are given a Codeforces account and $n = 6$ problems,
authored and prepared by [user:qwexd,2026-09-12] and [user:jeroenodb,2026-09-12].
</p>
<p>
In one operation, you may choose a problem $i$ ($1 \le i \le n$)
and submit a program intended to solve it.
</p>
<p>
The maximum scores are given below. Problem E is divided into two subtasks.
</p>
<div style="overflow-x:auto; margin:0.5em 0 1em;">
<table style="border-collapse:co...
We would like to thank:
Announcement of Codeforces Round 1121 (Div. 2)
44.
Codeforces Round #633 **내가 돌아왔다!** (Hello, Codeforces!)
I am thrilled to introduce you to [Codeforces Round #633](https://codeforces.me/contests/1338,1339). Followings are basic information:
- This contest will take place on [contest_time:1338].
- The round is **rated for all participants** who can understand this announcement. There will be two divisions.
- There are **5 problems** in each division and you will have **2 hours** to solve it.
- Score distribution will be announced later.
Followings are contributors:
- **Authors**: [user:McDic,2020-04-10] (All Div.2 problems and Div.1 D), [user:tzuyu_chou,2020-04-10] (Div.1 E)
- **Coordinator**: [user:antontrygubO_o,2020-04-10]
- **Testers**:
- **Phase 1** (When this round was supposed to be Div.2): [user:dorijanlendvaj,2020-04-10], [user:SoulTch,2020-04-10], [user:pajenegod,2020-04-10], [user:AryaPawn,2020-04-10], [user:Rahul,2020-04-10], [user:mcdx9524,2020-04-10], [user:300iq,2020-04-10], [user:Nemo,2020-04-10], [user:tfg,2020-04-10]...
[user:MikeMirzayanov,2020-04-10] and his team for maintaining Codeforces and
Polygon. I wouldlike to say same sentence, ptz camp with higher constraints._ - _I don't like it._ - _Rock Scissor Paper
makes, ] and his team for maintaining Codeforces and Polygon. I would like to say same
sentence again as I
Announcement of Codeforces Round 633 (Div. 1)
Announcement of Codeforces Round 633 (Div. 2)
45.
My Reply on "Mike, please stop ignoring your userbase" This is a response to [this post](https://codeforces.me/blog/entry/141442).
Originally, I intended to leave it as a comment, but the text turned out to be quite long. So I decided to publish it as a separate post.
Hi!
Let me start with the main idea: I don't think I ignore too many issues (though I do ignore some). In most cases, I do work on them, but these days I just don't feel like writing about it. That kind of communication takes a lot of energy — which I currently don't have.
Oh, it looks like you edited the beginning of the blog, and now it triggers less of an internal protest in me. Honestly, lately I've been in
a difficult emotional state, and I just don't have the energy or desire to interact much. It's not just about Codeforces — it's generally harder for me to engage than it used to be. I won't go into details (health, relocation, family issues, changes in the nature of my work due to bots) — all of that affects my mental state and willingne...
(though I do ignore some). In most cases, I do work on them, but these days I
just don't feellike, change and the code or settings need to be adapted, we do new things (like the
recent blitz round, for me in such a situation. Looks like the team investigated and added updates
to the announcement, violations — maybe that's why somehow I skipped it. The round in question
soundslike it had major, ). In most cases, I do work on them, but these days I just don't feel like
writing about it. That kind, > MikeMirzayanov did make a comment about this api 3 years ago. However, it
seemslike there was no, I've often heard suggestions like “let's add SMS verification” or even more
extreme ideaslike, Oh, it looks like you edited the beginning of the blog, and now it triggers
less of an internal
46.
[Tutorial] Generating Functions in Competitive Programming (Part 1) Hi everyone! Inspired by the recent [Codeforces Round 641](https://codeforces.me/contest/1349), I decided to write an introductory tutorial on generating functions here. I am by no means an expert in generating functions so I will write about what I currently know about them. [user:MiFaFaOvO,2020-05-15] has written a really interesting [blog](https://codeforces.me/blog/entry/76447) here on Codeforces about more advanced applications of generating functions, but I think there is no English tutorial on the basics of this topic yet (or at least on CP sites). Thus, I would like to share about this topic here.
I plan to split this tutorial into two parts. The first part (this post) will be an introduction to generating functions for those who have never learned about them at all, and some standard examples and showcases of generating functions. The second part will be a collection of several applications of generating functions in CP-style problems. If you are already familiar with ge...
polynomial operations in $O(n\log n)$ time or $O(n\log^{2} n)$ time like
$\sqrt{P(x)}$, $\ln(P(x, tutorial on the basics of this topic yet (or at least on CP sites). Thus, I
wouldlike to share about, Note: Don’t we need to care about convergence issues like $|x|<1$? Well, it
depends on what you, We can manipulate generating functions very much like how we would manipulate
other algebraic
47.
Linear Recurrence and Berlekamp-Massey Algorithm **#IjustWantContribution**
It seems there isn't any blog about Berlekamp-Massey Algorithm around here, so I decided to go on a try. :P
Acknowledgement: Hats off to matthew99 for introducing this algorithm.
#### What is 'linear recurrence'?
Assuming there is a (probably infinity) sequence $a_0,a_1...a_{n-1}$, we call this sequence satisfies a linear recurrence relation $p_1,p_2...p_m$, iff $\forall i \geq m,~a_i=\sum_{j=1}^m a_{i-j}p_j$. (Obviously, if $m\geq n$ any $p$ can do :P)
#### How to calculate k-th term of a linear recurrence?
For a polynomial $f(x)=\sum_{i=0}^{t} c_ix^i$, we define $G(f)=\sum_{i=0}^{t} c_ia_i$.
Obviously $G$ satisfies $G(f)\pm G(g) =G(f \pm g)$.
Because $a_i=\sum_{j=1}^m a_{i-j}p_j$, if we let $f(x)=x^m-\sum_{j=1}^m x^{m-j}p_j$, then $G(f)=0$. Also $G(fx),G(fx^2)...=0$. So $G(fg)=0$ (g is any polynomial).
What we want is $G(x^k)$. Because $G(f\lfloor x^k/f\rfloor)=0$, then $G(x^k)=G(x^k \bmod f)$.
[cut]
We can calculate $x^k \b...
]$. The transitions of $f$ is something like $f[i][j]=\sum_{k=1}^m
f[i-1][k]v[k][j]$. In old days
48.
On AI ruining "solving math problems with computer" You all went crazy.
Instead of arguing with every opinion separately, I just want to ask one question: Why are you doing competitive programming? Is it to get to 1600 rating and put it on the resume? If so, please use AI, cheat, and do everything possible to get to 1600 as soon as possible and get the f**k out of this platform. And if you are (not) a normal person and do competitive programming because it is fun to solve problems, do you think it is fun to copy the problem statement into an AI model prompt and then copy the code it spews to submit? And why do you assume that everyone else will do that if there will be such an opportunity? The same goes for cheaters. Yes, some people do not do this as a sport, for fun. Why do you care?
I do think that AI is ruining competitive programming. By proxy. And that proxy is all of you who are running around yelling "We will all die, somebody do something about AI". I have seen a couple of comments saying "authors/coordinators must make s...
for us to enjoy. Lately, I feel like we (as a community) were focusing too
much on the negatives, remember that this was supposed to be a fun activity for us to enjoy. Lately,
I feellike we (as a
49.
Pro Tips - get them while they are free More tips to use while studying under my tutoring [here](https://um-nik.notion.site/Pro-Tips-8b923b0dc7c94770824bdf4184afc43c).
## Stop caring about the rating
Unless there are prizes involved, your position in the standings makes no difference. I can feel awful after winning a contest if I know that I could solve one more problem. Or I can feel OK losing a bunch of rating points knowing that I solved everything I could. Because rating is just a number. It is highly volatile and it depends on other people, not only on your skill. But solving problems is totally up to you. And you should evaluate yourself not based on some random metric, but on your honest feeling whether you performed up to your expectations. Your sense of self should be under your control, don't get hung up on some imaginary value, you cannot reduce your progress to a single number.
## Don't use more than one account
Rating is just a number. Do not be afraid to lose your colour. You’ll get it back in no t...
problems, which is not actually random, but the problems you subconsciously
didn’tlike. By upsolving you, - What could I have done better? - What areas took me more time than I would
like? - Are there
50.
New Year Gift: Codeforces Catalog — Early Access <img style="float:right; margin: 1em 0 0 0; width:200px;" src="/predownloaded/13/56/1356aec5e821992f4dc1d1c8b297917eae36d5be.jpg"/>
Hey!
Are you celebrating the New Year already? I wish you a Happy New Year. I wish you progress, joy and health.
We decided to show the current progress in the catalog of significant Codeforces posts.
So far, quite a bit has been implemented, but somehow you can already use it! Would you like to see it? Follow the link:
<br/>
<div style="font-size:30px;"><a href="https://codeforces.me/catalog">Catalog →</a></div>
<br/>
As planned, the catalog will collect and classify interesting and useful posts from Codeforces. Indeed, many useful articles have been written in Codeforces over 12 years (God, already 12 years!).
The catalog will be a kind of Codeforces golden fund in the future.
The catalog is a tree of topics to which posts can be added. We will still work on its functionality, for now there are only basic features. Don't ...
you can already use it! Would you like to see it? Follow the link:
, So far, quite a bit has been implemented, but somehow you can already use it!
Would youlike to see
51.
Codeforces Round 923 (Div. 3) Gamarjoba, Codeforces!
On [contest_time:1927] will start [contest:1927], the next Codeforces round for the Div.3.
Lately, I've been coming up with problem ideas less frequently, but I don't want to lose this skill.
Welcome to the round where all problems are my own creation! I hope you'll enjoy them.
A huge thank you to [user:Vladosiya,2024-02-05] for preparing the majority of problems in Polygon. Also, thanks to [user:pashka,2024-02-05] and [user:KAN,2024-02-06] for helping with the discussion of problem ideas.
Thank you very much [user:74TrAkToR,2024-02-05], [user:CLown1331,2024-02-05], [user:EternalAlexander,2024-02-05], [user:Jostic11,2024-02-05], [user:Killever,2024-02-05], [user:KoT_OsKaR,2024-02-05], [user:LoveWX,2024-02-05], [user:MADE_IN_HEAVEN,2024-02-05], [user:dan_dolmatov,2024-02-05], [user:jnmtz111__,2024-02-05], [user:pedrolino,2024-02-05], [user:theRealChainman,2024-02-05], [user:yorky,2024-02-05] for testing the round.
As usual for the Div.3 rounds:
...
will be rated for you. Good luck to all! I really like it when the round
announcement, I really like it when the round announcement includes a photo of the authors. I
plan to add
Announcement of Codeforces Round 923 (Div. 3)
52.
Codeforces: summing up 2016 year <center>
<img src="http://assets.codeforces.com/images/Happy-New-Year-HD-Wallpapers.jpg"/>
</center>
Somehow it turns out that for me, each coming the New Year means more and more. This is an excellent opportunity to take stock, to breathe fresh air and to go through a kind of catharsis. This is a rare opportunity to be with the family, dress up in Santa Claus and just have fun.
Holidays give me a clear criterion that I do the right things. If during the holidays, I begin to think about it, to recall some deferred ideas, eager to implement something new. So I’m doing right things. My things. And now exactly the same as in several recent years: during New Year holidays I write code for Codeforces, trying new technology or simply I’m making minor changes in testlib. It is necessary to me: at this moment are no deadlines, do not press the obligation. Just now, at 3 am, I can not break away from being able to do something new. I would like to take a moment and realize that spun in ...
can not break away from being able to do something new. I would like to take a
moment and realize
53.
Support Anton Trygub Since [user:antontrygubO_o,2020-07-12] has became CF coordinator he is trying hard to make rounds which meet his standards for quality and (more importantly) beauty. I think he is doing a great job.
For some time there is a very vocal group of people who dislike his view on things. I feel like it will be hard for Anton to maintain his work if he would think that majority of people doesn't approve.
But I don't really think that this is a majority. If you enjoy rounds prepared or coordinated by Anton, please comment here with some positive feedback.
`#AdHocProblemsMatter`
. For some time there is a very vocal group of people who dislike his view on
things. I feellike, For some time there is a very vocal group of people who dislike his view on
things. I feellike it
54.
Codeforces Global Round 12 <img src="/predownloaded/2e/e2/2ee2f0f95f05aae6ca6f47812dae2dc2aabf277b.png" style="width:200px; float:right; margin: 0 1em 1em 1em"/>
¡Buenos días! (That's Spanish for "what's up homies")
On [contest_time:1450] we will host [contest:1450].
It is the sixth round of a 2020 series of [Codeforces Global Rounds](/blog/entry/65002). The rounds are open and rated for everybody.
The prizes for this round:
- 30 best participants get a t-shirt.
- 20 t-shirts are randomly distributed among those with ranks between 31 and 500, inclusive.
The prizes for the 6-round series in 2020:
- In each round top-100 participants get points according to the table.
- The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
- The best 20 participants over all series get sweatshirts and place certificates.
Thanks to XTX, which in 2020 supported the global rounds initiative!
The problems were written and prepared by smart Cu...
We would like to distribute our thanks equally to the following people who made
this round possible.
Announcement of Codeforces Global Round 12
55.
Codeforces Round #663 (Div. 2) Henlo Codeforces! ^_^
I invite you to participate in [Codeforces Round #663 (Div.2)](https://codeforces.me/contests/1391) taking place on [contest_time:1391]. The round is rated for users rated less than 2100, while other users can participate non-competitively.
The round features five problems, and you have 2 hours to solve them. There may, or may not, be an interactive problem; regardless, you should [know](https://codeforces.me/blog/entry/45307) how to deal with them.
I would, now, like to thank–
- [user:antontrygubO_o,2020-07-31] for coordination <s>and manipulation</s>.
- [user:AwakeAnay,2020-07-31], [user:AsleepAdhyyan,2020-07-31], and [user:RestingRajarshi,2020-07-31] for always listening to my problem ideas.
- [user:BRCode,2020-08-03] for making [3b1b-style](https://www.youtube.com/watch?v=LYrtRnz35sU) video editorials of the problems!
- [user:Aggu_01000101,2020-08-07], [user:Devil,2020-07-31], [user:Dragnoid99,2020-07-31], [user:NOOBxCODER,2020-07-31], [use...
I would, now, like to thank–
Announcement of Codeforces Round 663 (Div. 2)
56.
About Division 4 Rounds Hello Codeforces!
Initially, here was an announcement of the first round for the fourth division. But currently, in comments there is the huge discussion of division 4 rounds in general. Honestly, I read all (almost all) comments.
I decided to replace the text of this post to match the comments better. Also, I'll write one more post separately to discuss problems. And this post will be about division 4 rounds in general.
About Div4 rounds:
* I'm not afraid of a queue or servers overload: it is technical challenges and my task is to resolve them. The previous biggest rounds were not overshadowed by technical problems and judging delays, they went well. I see no reasons why something should work much worse in the rounds for Div. 4 (and if it does, then this should be fixed).
* I don't think that Div. 3 rounds are too dificult. Actually, they are quite good for beginners. They are noticeably easier than Div. 2. Almost every round trusted participants solve all problems, ma...
plan to use absolutely A+B level problems even for the Div. 4. I think
problems of levellike Div3A, * I'd like to repeat: Div. 3 and Div. 4 rounds doesn't affect the rate of Div.
1 and Div. 2 rounds, * Now I don't like the idea to run in parallel Div. 3 and Div. 4 rounds (like
we do for Div. 1 and, like the idea to run in parallel Div. 3 and Div. 4 rounds (like we do for Div.
1 and Div. 2). Div. 3
57.
Good Bye 2020 Hello Codeforces!
We have a pleasure to invite you to [Good Bye 2020](https://codeforces.me/contests/1466), which will take place on [contest_time:1466]. You will have **3 hours** to solve **9 problems**. The round will be rated for participants of both divisions.
The problems for this round were prepared by [user:gawry,2020-12-28], [user:Anadi,2020-12-28] and me.
We would like to thank everyone who made this round possible:
- [user:Nebuchadnezzar,2020-12-28] and [user:KAN,2020-12-28] for the excellent coordination;
- [user:H4CKOM,2020-12-28], [user:221,2020-12-28], [user:manikjain4,2020-12-28], [user:namanbansal013,2020-12-28], [user:Prabowo,2020-12-28], [user:saurabhshadow,2020-12-28], [user:Gauravvv,2020-12-28], [user:Fly_37,2020-12-28], [user:Thaid,2020-12-28], [user:__rkm__,2020-12-28], [user:6520,2020-12-28], [user:Himanshu_77,2020-12-28], [user:H_M_Vaghasiya,2020-12-28], [user:Jellyman102,2020-12-28], [user:SecondThread,2020-12-28], [user:Origenes,2020-12-28], [...
We would like to thank everyone who made this round possible:
Announcement of Good Bye 2020
58.
How to practice Competitive Programming [Um_nik version] CP is about solving problems fast. And as absurd as it may sound, I believe that <span style="color:blue">SOLVE</span> and <span style="color:red">FAST</span> are very different and almost independent parts, and you need to practice them separately.
Let’s look at some <span style="color:red">contest</span>, like a CodeForces round. For the sake of simplicity let’s assume that every problem has some *difficulty*, which is a numerical value denoting how hard it is, bigger values correspond to harder problems (it is not true, but it is an ok-ish approximation, at least if we consider subjective difficulty for a fixed person). Contests are made for a wide range of participants, and problemsetters strive to make contests interesting for a wide range of participants, which means having a *smooth difficulty gradient*. Well... as smooth as it is possible with 5-6 problems.
<spoiler summary="Graph 1">
I don't think I agree with this. To begin with, I don't think that having Data Structure problem is a requirement for a good contest at all, not just in first few positions.
However, for positions D2A-D2D, I just don't see a way to properly include data structure problems. Take some Data Structure problem, it consists from two parts:
<ul>
<li> Knowing/implementing the Data Structure </li>
<li> Actually thinking about the problem, and how this Data Structure...
problems and rounds I coordinate, and I would like to make a few points., **3.** While I understand that many people really like these topics and want
them to appear as, **7.** I would like to thank:
60.
Announce of Looksery Cup 2015 Hi, Codeforces!
<img src = "https://pp.vk.me/c622617/v622617180/32cdb/fCl4Wp0ouUM.jpg" style="float:right;width:18%;height:18%"/>
I am glad to announce Looksery Cup 2015, that is prepared by our developers, each one of them has made a great impact: [user:Sfairat,2015-06-05], [user:olpetOdessaONU,2015-06-05], [user:Sklyack,2015-06-05], [user:MrDindows,2015-06-05], [user:Rubanenko,2014-08-30], [user:Krasnokutskiy,2015-06-05], [user:2222,2015-06-05], [user:MaximM,2015-03-05], [user:Avalanche,2015-06-05], [user:Igor_Kudryashov,2015-06-05], [user:Kepnu4,2015-06-05] and I. Special thank to coordinator [user:Zlobober,2015-06-05] for the help with problems and advices and to [user:Delinur,2015-06-05] for the translation.
We've prepared 8 problems for you, that will have **random** order at the contest. Round will last 2 hours 30 minutes under the standard Codeforces rules with smooth dynamic scoring. We hope, that you will enjoy competition, and we will receive small amount of clars ...
Also I would like to thank [user:MikeMirzayanov,2015-06-05] and everybody, who
works on Codeforces
Announcement of Looksery Cup 2015
61.
Unimportant Um_nik Update Hello Codeforces! I have some big news.
I am trying to fight depression and live a fuller life. That includes some self-realization, maybe even (I don't even want to think about it) a job. I love cp, and I love teaching, and I know I already tried that, but I believe that this is the way.
With the help of my wife, I started to work on some related projects, and today we want to share them with the world. All of them are in the early stages, but we set a hard deadline for March 1, so...
Um
==================
Um... I am starting a series of educational YouTube videos / selected problems.
Every episode of Um is focused on one problem. I will explain how to solve this problem and talk about some related concepts. The idea is to show off some cool concepts using real problems.
The kicker: the problem is made public at least a week before the episode airs. So you can use Um as a source of problems. I will try to choose nice problems that highlight some ideas, and problems ...
[Discord server](https://discord.gg/HPjyarXvsN) and maybe follow the Um
series. If youlike my, can ban you — what's not to like?
62.
Codeforces New Feature: Rewrite Statements in Mashups Hello Codeforces!
There is something new for you. It seems that now it will be easier to reuse problems from the archive for educational and other purposes.
As you know, problems from past Codeforces rounds can be added to <a href="https://codeforces.me/blog/entry/10099">mashups</a> simply by their codes of the form like <code>1234D</code> (contest ID + problem letter). I myself regularly used this when I taught at Saratov University — it was very convenient to prepare trainings: problems have expected complexity and topics, there are Russian and English statements, you can see the analysis and many solutions. Because of hacks and uphacks, problems in the archive usually have strong tests. Almost all of them are well-prepared. In addition, with the help of <a href="https://codeforces.me/blog/entry/21912">user lists</a>, you can easily pick up problems for a group of students that they did not solve.
<img src="/predownloaded/2f/ec/2fecbf1f7e7a8f8655f0f74576fd1f6dbbc2307...
by their codes of the form like 1234D (contest ID + problem letter). I myself
regularly, /blog/entry/10099">mashups simply by their codes of the form like 1234D
(contest ID
63.
Who sorry_dreamoon really is Hello everybody. So I was really bored tonight and looked at so many comments being like "[user:sorry_dreamoon,2015-02-18] is X" and then people arguing and disagreeing. Since I had nothing better to do, I decided to finally unravel the truth of who he actually is. Here is the full case and how I actually got to him, hope you enjoy! :D
Selection
------------------
Let's start by the obvious. He managed to get first place in Div2 and he got second in Div1 (would've been first if the scores were static). So what we know is that he is really good.
Let's look at the past 10 Div1 contests. It turns out that only 8 times it happened that someone with a rating of less than 2500 got in top5. This is 8 times out of 50 people being in top5. But he was second and almost first, so he is better than that.
It turns out actually, that in the past 10 Div1 contests, only 2 times it happened that someone with rating less than 2500 got in top3.
Combining this information with [user:sorr...
Hello everybody. So I was really bored tonight and looked at so many comments
beinglike
64.
On Problems vs. Questions in Competitive Programming: My solution _TL;DR: The most reasonable solution to the problems vs. questions debate is to use none of the currently accepted terms._
As we have seen many times, particularly over the past few days, the task of assigning a label to the activities we perform as competitive programmers is a bigger problem than one might think at first. Time and time again we have seen people questioning what we should call our activities, often to the point of eliciting some rather harsh responses.
To give some context to this discussion, I will focus on two of the terms that members of the community have adopted over the years, _problems_ and _questions_. (Sorry task lovers! You have some fine points for sure, we frequently talk about subtasks rather than subproblems or subquestions after all :P). All quotations here are posted anonymously, but if you want to claim ownership of one of them or have one removed, just tell me. If you are familiar with the arguments, feel free to skip to the final section of the...
algorithmic problems. Question is meant to be shorter and in some sense more
lightweightlike "could you, it is used, and I personally believe that dictionaries and the like should be
descriptive rather, problemset. Point is, the other meanings associated to words matter, and for
somethinglike this the, shorter and in some sense more lightweight like "could you pass me the salt?"
or "why is the sky, > Quiz sounds like something that can contain questions, because you can answer
them. There can, > Wat? No, it doesn’t work like that. People do grammar mistakes, but I’ve
noticed that this one is
65.
Competitive Programming Hall of Fame — cphof.org Hi Codeforces!
Today I'd like to introduce the new [Competitive Programming Hall of Fame](https://cphof.org) project that is available at [cphof.org](https://cphof.org)
#### Background
A long time ago I've noticed that there is no single source of information with the results of previously held large programming competitions. Moreover, plenty of older championships don't maintain their websites anymore, and it becomes hard or even impossible to find any details about them, which is pretty sad. So I've decided to create [Competitive Programming Hall of Fame](https://cphof.org).
#### About the project
Competitive Programming Hall Of Fame is a project created to preserve the history of international championships in competitive programming. This website collects the results of the final stages for both worldwide and regional contests. By international championship we mean a contest consisting ...
reimbursement for finalists, and no restrictions except for legal ones, like
age 18+ for finalists, . Without them our website couldn't have as much detailed data as it has today.
Also, I wouldlike to say, Hi Codeforces! Today I'd like to introduce the new [Competitive Programming
Hall of Fame](https, Today I'd like to introduce the new [Competitive Programming Hall of
Fame](https://cphof.org
66.
Goodbye 2022. Hello 2023! <img src="/predownloaded/f4/1f/f41ff0beb745fb2f84a55d9f2cc451273efc6267.jpg" style="width:300px; float:right; margin: 0 1em 1em 1em"/>
Dear Santa Claus. Please make 2023 a happy year, not just that. Please, we all need this.
Hello, 2023!
And we have traditional gifts!
#### <img src="https://assets.codeforces.com/images/wand.png"/> Change Handle Feature
Hurry! Only until the 10th of January, you can change your handle (<b>but only once</b>)! Note that it will be possible to roll back the changes or change the handle again only _after a year_. Be careful what you wish for.
You can change your handle to the new one which wasn't used before by anybody or which was used by you before. The links to a profile page with an old handle would automatically redirect to the actual profile.
Again, this year if you took part in at least 10 rounds you can request a handle of an _inactive_ participant. It means that the participant should have a period of activity on Codeforces
of...
something with it. Especially, I do not like requests like "this is my second
account, I took an, with it. Especially, I do not like requests like "this is my second account, I
took an interesting
67.
Codeforces Round #685 (Div. 2) Hi everyone!
I would like to invite you to another one of our rounds, that I set with my friends [user:Jeel_Vaishnav,2020-11-19], [user:FastestFinger,2020-11-19], [user:Utkarsh.25Dec,2020-11-19], [user:the_hyp0cr1t3,2020-11-19] and [user:ridbit10,2020-11-19]
We had two approved contests, but decided to merge it into one with more logical thinking ^_^
The round [contest:1451] that will take place on [contest_time:1451]. If your rating is less than **2100**, this round will be rated for you; otherwise, you can participate out of competition.
I would really like to thank my co-setters and:
- [user:300iq,2020-06-05] for coordinating our round and always replying quickly.
- [user:jtnydv25,2020-11-19], [user:antontrygubO_o,2020-11-20], [user:Monogon,2020-11-19], [user:Vivek1998299,2020-11-19], [user:tejas10p,2020-11-19], [user:smit_mandavia,2020-11-19], [user:T1duS,2020-11-20], [user:kshitij_sodani,2020-11-20], [user:ayushsinghiitr,2020-11-19], [user:Ajax,2020-11-19], [user:...
Hi everyone! I would like to invite you to another one of our rounds, that I
set with my friends, I would like to invite you to another one of our rounds, that I set with my
friends, I would really like to thank my co-setters and:
Announcement of Codeforces Round 685 (Div. 2)
68.
Codeforces Round #884 (Div. 1 + Div. 2) Hello, Codeforces!
I invite everyone to participate in [contest:1844], which will start on [contest_time:1844]. The round is a combined round and will be rated for everyone.
You will be given **8 problems** and **3 hours** to solve them. One of the problems is divided into two subtasks. The scoring distribution will be:
$500$ — $1000$ — $1250$ — $1500$ — $2000$ — $(2000+1000)$ — $3500$ — $4000$
All problems are written and prepared by me. I would like to thank:
* [user:irkstepanov,2023-07-10] for carefully coordinating the round.
* [user:MikeMirzayanov,2023-07-10] for the wonderful Polygon and Codeforces platforms.
* [user:4fecta,2023-07-10], [user:ak2006,2023-07-10], [user:amethyst0,2023-07-10], [user:AndreySergunin,2023-07-10], [user:BalintR,2023-07-10], [user:ChrisT,2023-07-10], [user:Div1D4Life,2023-07-10], [user:dorijanlendvaj,2023-07-10], [user:Dormi,2023-07-10], [user:errorgorn,2023-07-10], [user:Golovanov399,2023-07-...
All problems are written and prepared by me. I would like to thank:
Announcement of Codeforces Round 884 (Div. 1 + Div. 2)
69.
Codeforces Round #668 Hello Codeforces!
We are glad to invite you to [contest:1404] and [contest:1405], which will take place on [contest_time:1404].
The problems were created by [user:Ari,2020-09-04], [user:Kuroni,2020-09-04], [user:Maripium,2020-09-04], [user:Monogon,2020-09-04], [user:antontrygubO_o,2020-09-04] and [user:hugopm,2020-09-04].
We would like to thank:
- [user:dorijanlendvaj,2020-09-04], [user:Jatana,2020-09-04] and [user:MrDecomposition,2020-09-04] for helping with the preparation of the tasks.
- [user:gamegame,2020-09-04] for always being nice and supportive.
- [user:DatVu,2020-09-04], [user:Maripium,2020-09-04], [user:MofK,2020-09-04] and [user:atoiz,2020-09-04] for stealing the round #666, forcing us to retheme around ponies.
- [user:Monogon,2020-09-04] for making the author list more colorful.
- [user:TwentyFour,2020-09-04], [user:thenymphsofdelphi,2020-09-04], [user:qlf9,2020-09-04], [user:mcfr,2020-09-04], [user:Tlatoani,2020-09-04], [user:i_love_Lisa,2020-09-04...
hope you will like it., We would like to thank:
Announcement of Codeforces Round 668 (Div. 1)
Announcement of Codeforces Round 668 (Div. 2)
70.
A problem collection of ODE and differential technique ##A problem collection of ODE and differential technique
*This problem might be well-known in some countries, but how do other countries learn about such problems if nobody poses them.*
For those who are interested in well-known problems in China.
Thank [user:Elegia,2020-04-23] and [user:djq_cpp,2020-04-23] for developing this technique. Thank [user:tEMMIE.w.,2020-04-23] for reviewing this article.
####[Chain Reaction](http://uoj.ac/problem/50) in UOJ Round 3 By [user:vfleaking,2020-04-23]
**Statement**
You are given a set $A$, you need to compute $g_{i} = \frac{1}{2} \sum_{j,k}{i-1 \choose j}{i-1-j \choose k} g_jg_k$ where $i-1-j-k \in A$.
**Solution**
Let the EGF of $g$ be $x(t)$ and EGF of $A$ be $a(t)$. Thus $x'(t)=\frac{1}{2} a(t) x^2(t)+1$. We can solve this equation by D&C and FFT in $O(n\log^2 n)$. But there is a <s>slower</s> solution in $O(n\log n)$.
For a polynomial equation $f(x(t))=0$, we can use the Newton's method to solve it. If we find ...
There are many similar problems like [Lucky
Tickets](https://codeforces.me/problemset/problem, multiplicities. Since it's completely additive like $\ln n$. A rigorous proof
can belike that we define a, something like $P(H)\cdot A=(P(H))' \cdot B+(P(H))'' \cdot C$, where $A,B,C$
are three polynomials
71.
Hello 2026 Happy new year, Codeforces!
We are glad to invite you to take part in [contest:2183], which will start on [contest_time:2183]. You will be given **9 problems** and **3 hours** to solve them. **At least one problem will be interactive**, so please make sure to read the [guide for interactive problems](https://codeforces.me/blog/entry/45307) before the contest. This round is open and rated for everyone.
Problems are authored by [user:wangmarui,2025-12-23], [user:Gold14526,2025-12-23], [user:Hoks_,2025-12-23], [user:Neil_Qian,2025-12-23], [user:Network_Error,2025-12-23], [user:jiazhichen844,2025-12-23] and [user:xvchongyv,2025-12-23].
We would like to thank:
- [user:Um_Nik,2025-12-23] for pre-reviewing the round.
- [user:Proof_by_QED,2025-12-23] for his wonderful coordination.
- [user:Alexdat2000,2025-12-23] for Russian translation.
- [user:Geothermal,2025-12-23], [user:StarSilk,2025-12-23], and [user:Dominater069,2025-12-23] for nutella testing.
- [user:Mindeve...
We would like to thank:
Announcement of Hello 2026
72.
Blogewoosh #6 Hello, codeforces!
Sorry for the long break, but the last weeks of holidays and the first weeks of academic year took my attention. I hope today's trick will make you forgive me. :P
I invented this trick a few years ago, but for sure I wasn't first, and some of you already know it. Let's consider the following **interactive** task. There are $n$ ($1 \leq n \leq 10^5$) hidden integers $a_i$, each of them from range $[1, 10^{18}]$. You are allowed to ask at most $103000$ queries. In one query you can choose two integers $x$ and $y$ ($1 \leq x \leq n, 1 \leq y \leq 10^{18}$) and ask a question ''Is $a_x \geq y$?'' The task is to find the value of the greatest element in the hidden array. The checker **isn't** adaptive.
Unfortunately, this task is only theoretical, and you cannot solve it anywhere, but it'll turn out, that solution can be handy in many other, much more complicated problems.
[cut]
$ $
Even beginners should be able to quickly come up with a solution which ask...
lower values... but no, things like that would aim only at really weak, random
test cases. More, it, not — to skip the binary search. It gave me many more points (it was an IOI-
like problem) even, . Here comes an idea: let's iterate over numbers in the hidden array like in
the $n \cdot log(d, ... but no, things like that would aim only at really weak, random test cases.
More, it seems that
73.
New: Diagnostics of Solutions in C++ (Better UI) Hello,
Do you remember my post <a href="/blog/entry/55902">New: Diagnostics of Solutions in C++</a>? If not, please read it.
Recently, I've implemented better UI to show you diagnostics. Now in on the status pages sometimes you can notice something like this:
Click to the  and you'll see the code with the highlighted problematic line and description with the possible reason of the mistake.
Do you like it?
you can notice something like this:, Do you like it?
74.
Codeforces Round #666 
(Translation: Hello)
We are very happy to invite you to take part in our contest, [contest:1396] and [contest:1397]. The contest starts on [contest_time:1396] and lasts **2 hours**. In both divisions, there will be **5 problems** for you to solve.
The problems were authored and prepared by [user:Maripium,2020-08-28], [user:DatVu,2020-08-28], [user:MofK,2020-08-28], and me.
We are very grateful and would like to sincerely thank the following people for their assistance in preparing the round:
- [user:300iq,2020-08-28] for coordinating our round and for reviewing and helping us with the problems.
- [user:minhtung04042001,2020-08-28], [user:TomOwO,2020-08-28], [user:EntityIT,2020-08-28] for their problem ideas and for helping us prepare the problemset.
- [user:codeveloppy,2020-08-28], [user:lolicon234,2020-08-28], [user:DeadlyCritic,2020-08-28], [user:growup974,2020-08-28], [user:VietCT,2020-08-28...
Finally, we would like to thank all of you for participating in the contest!, We are very grateful and would like to sincerely thank the following people for
their assistance in
Announcement of Codeforces Round 666 (Div. 1)
Announcement of Codeforces Round 666 (Div. 2)
75.
About Gift Distribution Hello everyone!
<img alt="Packages" src="https://codeforces.me/predownloaded/a7/ce/a7ce52909464af02e2f922990c8c7e4418b98f67.jpg" style="float:right; height: 500px; margin: 10px; max-width:50%;"/>
“My name is Una and I am from the Codeforces team” — this is usually how my messages to users of the site begin. One of the reasons why I write a message is very joyful — sending out gifts to the winners of the competitions and now also to participants of crowdfunding dedicated to the 10th anniversary of Codeforces.
That’s what I’d like to talk about a little bit.
Everyone knows where it all starts — there is a round with gifts, in the comments to which after the ending the coordinators or Mike post a list of winners.
Then, in a short time, all winners receive a system message asking them to update the data about the size of the T-shirt and address. In some special cases, such as crowdfunding, for example, all participants receive an email. For various reasons...
That’s what I’d like to talk about a little bit.
76.
Codeforces Global Round 10 Hi! <img src="/predownloaded/2e/e2/2ee2f0f95f05aae6ca6f47812dae2dc2aabf277b.png" style="width: 200.0px;float: right;margin: 0 1.0em 1.0em 1.0em;max-width: 100.0%;max-height: 100.0%;">
On [contest_time:1392] we will host [contest:1392].
This is the fourth round of a 2020 series of [Codeforces Global Rounds](https://codeforces.me/blog/entry/65002). The rounds are open and rated for everybody.
The prizes for this round are as follows:
- The top 30 participants get a t-shirt.
- 20 t-shirts are randomly distributed among those with ranks between 31 and 500, inclusive.
The prizes for the 6-round series in 2020 are as follows:
- In each round, the top 100 participants get points according to [this table](https://pastebin.com/QT5sXEaT).
- The final result for each participant is equal to the sum of the points they got in the four rounds where they placed the highest.
- The top 30 participants over all series get sweatshirts and place certificates.
Thanks to XTX for sup...
], [user:qlf9,2020-08-12], [user:golions,2020-08-12],
[user:MagentaCobra,2020-08-12], and me. We wouldlike
Announcement of Codeforces Global Round 10
77.
C++ Tricks I see lots of programmers write code like this one:
~~~~~
pair<int, int> p;
vector<int> v;
// ...
p = make_pair(3, 4);
v.push_back(4); v.push_back(5);
~~~~~
while you can just do this:
~~~~~
pair<int, int> p;
vector<int> v;
// ...
p = {3, 4};
v = {4, 5};
~~~~~
[cut]
[This](http://codeforces.me/blog/entry/10124) is a great C++11 tutorial for those who want to know more about C++11.
#### 1. Assign value by a pair of {} to a container
I see lots of programmers write code like this one:
~~~~~
pair<int, int> p;
// ...
p = make_pair(3, 4);
~~~~~
while you can just do this:
~~~~~
pair<int, int> p;
// ...
p = {3, 4};
~~~~~
even a more complex `pair`
~~~~~
pair<int, pair<char, long long> > p;
// ...
p = {3, {'a', 8ll}};
~~~~~
What about `vector`, `deque`, `set` and other containers?
~~~~~
vector<int> v;
v = {1, 2, 5, 2};
for (auto i: v)
cout << i << ' ';
cout << '\n';
// prints "1 2 5 2"
deque<vector<pair<in...
position. x is `unsigned int` and like previous function this function with
suffix 'l' gets a `unsigned, . x is `unsigned int` and like previous function this function with suffix 'l
gets a `unsigned, A raw string skips all escape characters like `\n` or `\"`. e.g., I see lots of programmers write code like this one:, Lambdas are like other languages' closure. It defines like this:, This function returns number of 1-bits of x. x is `unsigned int` and like
previous function this, This library includes many of libraries we do need in contest like `algorithm`,
`iostream`, `vector, When you work with STL containers like `vector`, you can use `move` function to
just move container, You already know literals from C++ like: `0xA`, `1000ll`, `3.14f` and so on...
78.
Codeforces Global Round 11 <img src="/predownloaded/2e/e2/2ee2f0f95f05aae6ca6f47812dae2dc2aabf277b.png" style="width:200px; float:right; margin: 0 1em 1em 1em"/>
Hi!
On [contest_time:1427] we will host [contest:1427].
This is the fifth round of the 2020 series of [Codeforces Global Rounds](https://codeforces.me/blog/entry/65002). The rounds are open and rated for everybody.
The prizes for this round are as follows:
- 30 best participants get a t-shirt.
- 20 t-shirts are randomly distributed among those with ranks between 31 and 500, inclusive.
The prizes for the 6-round series in 2020:
- In each round top-100 participants get points according to the [table](https://pastebin.com/QT5sXEaT).
- The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
- The best 20 participants over all series get sweatshirts and place certificates.
Thanks to XTX, which in 2020 supported the global rounds initiative!
Problems for this round ar...
Announcement of Codeforces Global Round 11
79.
Codeforces Global Round 13 Hello Codeforces!
On [contest_time:1491] we will host [contest:1491]. **<img src="/predownloaded/2e/e2/2ee2f0f95f05aae6ca6f47812dae2dc2aabf277b.png" style="width: 200.0px;float: right;margin: 0 1.0em 1.0em 1.0em;max-width: 100.0%;max-height: 100.0%;">**
It is the first round of a 2021 series of [Codeforces Global Rounds](https://codeforces.me/blog/entry/65002). The rounds are open and rated for everybody.
The prizes for this round:
- 30 best participants get a t-shirt.
- 20 t-shirts are randomly distributed among those with ranks between 31 and 500, inclusive.
The prizes for the 6-round series in 2021:
- In each round top-100 participants get points according to the [table](https://pastebin.com/QT5sXEaT).
- The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
- The best 20 participants over all series get sweatshirts and place certificates.
Thanks to XTX, which in 2021 supported the global rounds ...
We would also like to thank:
Announcement of Codeforces Global Round 13
80.
Mike, alts and unfriendly comments I know that the following content might not be welcomed and I will lose lots of contribution, but I should write something about such a bunch of chaotic comments under there.
I may delete this tomorrow if I find my words useless.
### What happened
Several months ago, Mike posted a blog about banning [user:orzdevinwang,2024-10-07]'s alt, [user:zh0ukangyang,2024-10-07], and more alts. Lots of participants, including myself, were angry because [user:zh0ukangyang,2024-10-07] is such a talented coder and codeforces didn't treat the rule of single account seriously at that time. Then more and more alt accounts with high rating are reported, including mine.
After that I didn't use alts anymore, and most of high-rated Chinese participants(100% in my friends) discarded their alts and followed the rules. We might think that the rules are not that perfect, but rules are rules. I believe that no one will say "Banning [user:zh0ukangyang,2024-10-07] is unacceptable".
But I know what y...
Mike makes you like a joker., looked like an outdated website today. But currently it still has the most
users, most contests, and
81.
Don't use rand(): a guide to random number generators in C++ Don't use `rand()`. Why? Let's jump right into some code. What value will the following code print, approximately?
~~~~~
#include <cstdlib>
#include <iostream>
using namespace std;
const int ITERATIONS = 1e7;
int main() {
double sum = 0;
for (int i = 0; i < ITERATIONS; i++)
sum += rand() % 1000000;
cout << "Average value: " << sum / ITERATIONS << '\n';
}
~~~~~
Should be about 500,000, right? Turns out it depends on the compiler, and on Codeforces it prints 16382, which isn't even close. [Try it out yourself](http://codeforces.me/contest/1025/customtest).
### What's happening here?
If you look up [C++ documentation on `rand()`](https://en.cppreference.com/w/cpp/numeric/random/rand), you'll see that it returns "a pseudo-random integral value between `0` and `RAND_MAX`." [Click again on `RAND_MAX`](https://en.cppreference.com/w/cpp/numeric/random/RAND_MAX) and you'll see that "This value is implementation dependent. It's guaranteed that th...
/Linear_congruential_generator). On the Codeforces compiler, it looks like this:, Both shuffles result in almost exactly $10^6$ average distance, like we
originally expected.
82.
New: Educational Rounds on Codeforces! Hi everybody!
First, I invite you to take part in an official [contest:597]. The fact is, the Codeforces team has made numerous changes to the platform (details are below), and we want to be sure that the basic functionality remained unchanged. This round will have a shortened duration of 1.5 hours, consist of 3 (maybe 4) problems that you might have already seen before. Its purpose is to test the system on the one hand, and on the other hand — to brighten up a Wednesday evening. Of course, the round will be unrated.
Now the main thing. This coming Friday (yes, the 13th of November) Codeforces starts another line of rounds. We called them Educational Rounds. Using my students at the Saratov State University Programmings Competitions Training Center as an example, I regularly notice that even those who have a considerable progress in the results on the rounds often have narrow purview in terms of standard topics and ideas, they are not familiar with many well-known problems ...
Their characteristics are like that:
83.
Hello, ITMO! <img src="https://assets.codeforces.com/images/itmo-logo.png" style="margin:1em 1em 1em 1em; float:right;"/>
Hi Codeforces!
For you, perhaps it was yet another Codeforces round. But not for me. [contest:1066] is the first round tested on new judging servers at ITMO University. And this is not just an update of location. Ta-dam! Now your solutions will be judged on the new Intel i3-8100 processors. And this is not all news. The number of judging servers has increased, which means fewer queues during rounds!
I am pleased to announce that now I live in St. Petersburg, I work at ITMO, and Codeforces is gradually moving from the walls of dear to me Saratov University to ITMO University. The decision to move was not easy for me. My plan is that, based on ITMO, I can focus more on Codeforces development and work on the platform. The number of world champions per square meter is simply overwhelming, and working with a large team of such enthusiasts (and professionals!) of sports pro...
Andreeve, I would like to say a personal thank you now. You have opened for me
an interesting world, programming, like me, is extremely inspiring. I always liked St. Petersburg
and the atmosphere of ITMO, working with a large team of such enthusiasts (and professionals!) of sports
programming,like me, is
84.
Good Bye 2018 The end of the December is fast approaching and it is time for the <strike>best</strike> last contest of the year! The Good Bye 2018 round will take place on [Sunday, December 30, 2018 at 14:35 UTC](https://www.timeanddate.com/worldclock/fixedtime.html?day=30&month=12&year=2018&hour=17&min=35&sec=0&p1=166).
As the people who already engaged in [discussions about the ultimate problem](https://codeforces.me/blog/entry/64025) suspect, I am the problem writer. As such, I'd like to thank to:
- [user:arsijo,2018-12-26] and [user:KAN,2018-12-26] for the round coordination and one of the problems,
- [user:AlexFetisov,2018-12-26], [user:cyand1317,2018-12-26], [user:Errichto,2018-12-26], [user:Jeel_Vaishnav,2018-12-26], [user:lewin,2018-12-26], [user:misof,2018-12-26], [user:winger,2018-12-26] for testing and invaluable suggestions,
- [user:MikeMirzayanov,2018-12-26] and everyone involved in building and running Codeforces and Polygon platform.
The round will last for 2:30 hours and...
://codeforces.com/blog/entry/64025) suspect, I am the problem writer. As such,
I'dlike to thank to:
Announcement of Good Bye 2018
85.
My strategy at AtCoder & CS Academy There's been much controversy lately about the late submission strategy not penalized by scoring systems of AtCoder and CS Academy.
Most of the relevant discussion happened earlier at http://codeforces.me/blog/entry/53431 and http://codeforces.me/blog/entry/53449.
I have a lot of thoughts on the topic, so I've decided to share them in a separate blog post.
I really like the strategic part of programming competitions. Of course, problem solving is more important.
But every contest consists of multiple problems, so there has to be a way of comparing participants which performed better at different problems.
There's a huge variety of scoring rules, and I find it truly amusing.
The "submit after solving all problems" strategy looks widely attributed to me now, mostly due to [user:Petr,2017-07-24]'s remarks regarding my participation in AtCoder contests.
In my opinion, it's wrong for multiple reasons.
A closer five-word description of my strategy is "implement after solvin...
submit when it feels like you should submit, not when the scoreboard tells
that you may submit., way. Like, when you come to an exam, you can either start working on the
problems one by one until, I feel like this strategy has only one major disadvantage. Time spent in the
beginning on problems, I really like the strategic part of programming competitions. Of course,
problem solving is more
86.
I've (also) completed Project Euler ###_I've been dreaming of writing this for so long._
---
About 2 years ago, [user:Radewoosh,2025-10-22] posted a [blog](https://codeforces.me/blog/entry/122793) celebrating that he had solved all 865 problems on Project Euler.
As a (naively optimistic) high-schooler back then, I thought to myself: _"Surely it can't be that hard, right?"_
2 years later, there are now 965 problems, and I've solved all of them!!
---
I can’t even begin to describe how happy I am, and how wonderful the journey has been. Maybe I’ll write about it in another blog sometime. For now, I’ll answer some questions you might be thinking...
####_Literally, who asked?_
No one, of course. But this achievement means a lot to me, and I hope that this post inspires someone out there.
####_How long did it take?_
Kind of a difficult question to ans...
For PE, there is no time limit, and there are no standard problems. Each new
problem islike a gift, Lastly, I'd like to thank the following people:
87.
Codeforces Round #300 (with prizes!) Hello, Codeforces!
[On Sunday, April 26th at 19:00 MSK](http://www.timeanddate.com/worldclock/fixedtime.html?day=26&month=4&year=2015&hour=19&min=0&sec=0&p1=166) the 300'th regular Codeforces Round will take place.
I would like to congratulate all Codeforces members and administration on this remarkable milestone. The platform has grown hugely in size and quality since its foundation, has hosted lots of exciting competitions, and has been providing the opportunity to everyone to hone their problem solving and algorithmic mastery. For this we thank the Codeforces platform creator [user:MikeMirzayanov,2015-04-24] and all the Codeforces crew. Keep up the incredible job, guys!
That being said, I'm excited to announce that the problems on the jubilee three-hundredth Codeforces Round will be set by me, Mikhail Tikhomirov ([user:Endagorion,2015-04-24]). You may remember the past rounds with my problems: [#99](http://codeforces.me/contests/138,139), [#109](http://codeforces.me/contest...
I would like to congratulate all Codeforces members and administration on this
remarkable milestone
Announcement of Codeforces Round 300
88.
How to sweep like a Sir ### Cutting to the chase
Clearly you don't need a PhD in Computing to sweep in the yard , but one might be useful in order to know linear and radial sweep algorithms. So , what's all about ? It's just what it sounds it is , sweeping linear ( up to down , for example ) or radial ( making a 360 degrees loop ).
How this can help? [cut] Well...
### Linear sweep
Suppose you a set of objects in an Euclidean plane. We need to extract information about some objects. The method of linear sweeping takes a line and moves it on a fixed direction. Usually the line taken would be vertical and the direction would be left to right or up to down.
Quite abstract for the moment , huh ? Let's go to a more specific example.
#### Rectangle union area
This example is well known. You have a set of rectangles with edges parallel to the OX and OY axes. What is their union area.
Well , first of all , let'...
How to sweep like a Sir, how to solve stuff like that if we have different Y coordinates for down
edges. Well , we'll keep, Basically , it would go like that:, Let's fix a point from the line. Now the line should rotate around this point ,
like in the picture, Now how to solve stuff like that if we have different Y coordinates for down
edges. Well , we'll
89.
About the Universal Cup, the Semifinals, and myself... Hey'all!
This gonna be a long post, so let me put an important TLDR here first: we are going to host **The 3rd Universal Cup Semifinals**, which will take place on August 24, 2025 **online**, at [11:00 (UTC)](https://www.timeanddate.com/worldclock/fixedtime.html?msg=The+3rd+Universal+Cup+Semifinals&iso=20250824T19&p1=33&ah=5). This contest will be a part of our Finals Team Selection, which the detailed rules and announcements will be published later next week.
---
Now for some extra context, if you're interested.
Last week, we hosted the [last online stage](https://codeforces.me/blog/entry/143954) of The 3rd Universal Cup. After that, I posted a [feedback form](https://docs.google.com/forms/d/1g5Txhiirh_lRsvGfkYWRozu0WDDGK31IAXjB7VgeVoM/edit) on our discord server. I received lots of thoughtful responses (P.S — it's still open, so feel free to fill it out if you haven't yet!).
I've read through all the feedback, and I'd like to share a few thoughts.
### On the Number...
CCF. Many amazing contests, like IOI, [JOIs](https://contests.ioi-jp.org/),
[POIs](https, other stages are nearly irrelevant. 2. If one strong team dominates a round,
like the things, season 2, we had several stages made with a mix of multiple "easy contests" (
like Chinese Provincial, I've read through all the feedback, and I'd like to share a few thoughts., If you would like to join us and win a place in the Finals, stay tuned for our
Semifinals! This, Ok, let us not end the post with all those shit memories. Like last season, our
next event will be
90.
About Problemsetting (for AtCoder and Codeforces) Since the amount of information available about the preparation of a competitive programming contest for AtCoder/Codeforces is very little, I decided to collect here what was my experience.
I will try to both describe my experiences and give some general advice to wannabe problemsetters.
I hope that this will be useful to future problemsetters who are "out of the loop". Moreover, participants might be curious to know what happens behind the scenes (and maybe the platforms may consider this as a form of constructive feedback).
*Acronyms*:
- AGC = Atcoder Grand Contest
- GR = Codeforces Global Round
### Why I know something about problemsetting?
I am in the competitive programming world since ~8 years: I have participated in IOI/ICPC/GCJ and a number of contests on AtCoder/Codeforces (and lately Codechef).
I am not a top participant but, being in this world for so long, I know, more or less, all the standard tricks.
Recently, I was the author of the *flagship conte...
and I kept sending problems until there were enough to create a problemset.
The process waslike this, expired, but something like 12 hours). During this time, me and
[user:rng_58,2020-12-09] double, in a contest is fine and what you don't like might be very cool for someone
else. On the other hand, that someone will not like your contest. It is also very likely that someone
will hate your contest, the AtCoder way of doing it. I like that Polygon makes it very hard to make
stupid mistakes (for, . Now the coordinator (and you if you like) will invite some testers to test
the problems. To, 1. I like to create problems. 2. I am proud and happy if the strongest
contestants in the world, The criterion I follow when accepting/rejecting my own problems is: would I like
to solve this, like I was wasting my time, drawing on a blank piece of paper, reading others'
problems to find, like this in the past) I will try to keep this in the recent blogs for some
days even if it does not
91.
I compiled a list of almost all useful blogs ever published on Codeforces [update: till 09.06.2021] <h3 style="color:red">If there are any blogs that I have missed, please tell in the comment section. Thank you.</h3>
# Mathematics Stuff
- [Number Theory in Competitive Programming [Tutorial]](https://codeforces.me/blog/entry/46620)
- [Number of points on Convex hull with lattice points](https://codeforces.me/blog/entry/62183)
- [FFT, big modulos, precision errors.](https://codeforces.me/blog/entry/48465)
- [Number of ways between two vertices](https://codeforces.me/blog/entry/19078)
- [Mathematics For Competitive Programming](https://codeforces.me/blog/entry/76938)
- [FFT and NTT](https://codeforces.me/blog/entry/19862)
- [Burnside Lemma](https://codeforces.me/blog/entry/51272)
- [Number of positive integral solutions of equation 1/x+1/y=1/n!](https://codeforces.me/blog/entry/76836)
- [On burnside (again)](https://codeforces.me/blog/entry/64860)
- [Simple but often unknown theorems/lemmas/formula? Do you know?](https://codeforces.me/blog/entry/55912)
- [Probabili...
convex is wrong?](https://codeforces.me/blog/entry/52341) - [How to sweep like
a Sir](https
92.
Codeforces Global Round 20 On [contest_time:1672], we will host [contest:1672].
**Note the unusual timing, it is 30 minutes earlier.** <strong><img src="/predownloaded/2e/e2/2ee2f0f95f05aae6ca6f47812dae2dc2aabf277b.png" style="width: 200.0px;float: right;margin: 0 1.0em 1.0em 1.0em;max-width: 100.0%;max-height: 100.0%;"></strong>
This is the second round of the 2022 series of [Codeforces Global Rounds](https://codeforces.me/blog/entry/65002). The rounds are open and rated for everybody.
The prizes for this round:
- 30 best participants get a t-shirt.
- 20 t-shirts are randomly distributed among those with ranks between 31 and 500, inclusive.
The prizes for the 6-round series in 2022:
- In each round top-100 participants get points according to the [table](https://pastebin.com/QT5sXEaT).
- The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
- The best 20 participants over all series get sweatshirts and place certificates.
T...
PS: We would like to take this opportunity to congratulate
[user:rotavirus,2022-04-20] or, We would like to thank the following people who made this round possible:
Announcement of Codeforces Global Round 20
93.
Codeforces Round #770 (Div. 2) Greetings to all humans and robots on this site!
[user:sevlll,2022-01-23], [user:crazyilian,2022-02-03], [user:Mangooste,2022-02-03], [user:imachug,2022-02-03] and me ([user:Alexdat2000,2022-02-03]) invite everyone to participate in the [Codeforces Round #770 (Div. 2)](https://codeforces.me/contests/1634), which will take place this [contest_time:1634]. **This round will be rated for all participants with a rating of strictly less than 2100.** You will have **2 hours and 30 minutes** to solve 6 problems. There will be an interactive problem in the round, so we recommend all new participants to read [Interactive Problems Guide](https://codeforces.me/blog/entry/45307).
The traditional thank-you list:
- Thanks to [user:antontrygubO_o,2022-02-03] for coordinating us for a very long time and helping to improve one of the tasks
- Thanks to [user:alexxela12345,2022-02-03] for tasks that did not survive till the final version of the round
- Thanks to [user:Pechalka,2022-02-03], [u...
Announcement of Codeforces Round 770 (Div. 2)
94.
Codeforces: giving extra time to participants Hello everyone!
Some of you are using Codeforces to host private onsite contests. When doing so, you may want to give extra time to a participant due to technical reasons (maybe they had a problem with their PC so they lost some time from the contest).
Previously it was only possible to increase contest duration for everyone, but now you can do this selectively.
Navigate to the contest registrants page, and you will see all the contestants and will be able to add extra time there, given you are the contest manager.
We would like to know if there are any issues with this new feature, so let us know if you encounter any.
PS: Also, similarly to [previously implemented feature for comments](https://codeforces.me/blog/entry/143022) you can now change your vote on a blog post within a minute.
We would like to know if there are any issues with this new feature, so let us
know if you
95.
[Tutorial] Collection of little techniques #### Introduction
There are a number of "small" algorithms and facts that come up again and again in problems. I feel like there I have had to explain them many times, partly because there are no blogs about them. On the other hand, writing a blog about them is also weird because there is not that much to be said. To settle these things "once and for all", I decided to write my own list about about common "small tricks" that don't really warrant a full tutorial because they can be adequately explained in a paragraph or two and there often isn't really anything to add except for padding.
This blog is partly inspired by [user:adamant,2022-03-15]'s [blog](48417) from a few years ago. At first, I wanted to mimic adamant's blog structure exactly, but I found myself wanting to write longer paragraphs and using just bolded sentences as section headers got messy. Still, each section is short enough that it would not make much sense to write separate blogs about each of these things.
A...
common in algorithms that look like this:, prime numbers like this:, It can be proven directly, but I like to think of it as a special case of
[Dilworth's theorem, Suppose you are solving a tree problem with dynamic programming. Your DP states
looklike this: for
96.
Good Bye 2022, Hello 2023! <img src="/predownloaded/cf/40/cf40c72e68b2aee5005a91f39e1d3404d917617b.jpg" style="float:right; margin:0 1em 1em 1em;"/>
Hello, Codeforces!
The year 2022 is finally coming to an end. Let's be clear. It's been a terrible year. Tomorrow I will make a wish, and you can easily guess which one.
I am writing these lines, and in a few minutes the Good Bye 2022 round will begin. And this inspires optimism and even pride in the community. Despite all these difficulties and troubles, we make rounds together and participate in them. Let's be grateful. And for this, let's say symbolic thanks to 2022 and a lot assistants: coordinators, problem writers, testers, participants and sponsors. We are many and we are together.
Can I have a little more detail on the Codeforces team?
Let me separately say thanks and words of admiration for the work of the coordinator of coordinators [user:KAN,2022-12-30] and the senior developer [user:geranazavr555,2022-12-30]. Both of you bring a lot to Cod...
I'd like 2023 to be the year we regret leaving exactly one year from now., Who I will not say thank you to, it's cheaters! Ugh, to be like that. Please
stay in 2022.
97.
Complex FFT is not as bad as you think ... Probably.
Hi everyone!
As some of you _might_ already know, I don't like NTT. Primary reasons for this are:
- I don't like modular arithmetic optimizations (Montgomery, etc).
- I don't like NTT mods, and prefer good, old $10^9+7$.
- I do like algebra of complex numbers.
But, to my regret, the overwhelming mainstream in modern competitive programming is NTT. Primary reasons are:
- It's allegedly faster.
- It needs less memory.
- It has no precision issues.
- People don't like algebra of complex numbers.
There is nothing I can do about the last, but today I'll address the first 3. Optimizing complex FFT was on my mind for quite some time, but I didn't really work on it _that much_ until the [blog](https://codeforces.me/blog/entry/142063) by [user:Qwerty1232,2025-04-24] on optimizing NTT has dropped, which motivated me to actually put some serious effort into this.
In this blog, we will primarily focus on optimizations that are specific to complex arithmetic...
people, like myself, there are much simpler and intuitive ways., , radix-4 FFT uses the following butterfly-like transforms:, - I don't like modular arithmetic optimizations (Montgomery, etc). - I don't
like NTT mods, and, . When doing regular FFT, we batch array elements in pairs, and apply the
following butterfly-like, As some of you _might_ already know, I don't like NTT. Primary reasons for this
are:, I would like to express my sincerest thanks to:, like algebra of complex numbers.
98.
Codeforces Global Round 4 <img src="/predownloaded/2e/e2/2ee2f0f95f05aae6ca6f47812dae2dc2aabf277b.png" style="width:200px; float:right; margin: 0 1em 1em 1em"/>
Hi!
On [contest_time:1178] we will host [contest:1178].
It is the fourth round of a new series of [Codeforces Global Rounds](https://codeforces.me/blog/entry/65002) supported by XTX Markets. The rounds are open for everybody, the rating will be updated for everybody.
You will be given 8 problems and 150 minutes to solve them. The scoring distribution will be **500-750-1250-1750-2000-(1500+1500)+3250+4000**.
The prizes for this round:
- 30 best participants get a t-shirt.
- 20 t-shirts are randomly distributed among those with ranks between 31 and 500, inclusive.
The prizes for the 6-round series in 2019:
- In each round top-100 participants get points according to the [table](https://pastebin.com/QT5sXEaT).
- The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
...
I would like to thank:
Announcement of Codeforces Global Round 4
99.
Pinely Round 3 (Div. 1 + Div. 2) <img src="https://assets.codeforces.com/images/pinely_logo.png" align="right" style="height: 200px;" alt="text"/>
Ciao, Codeforces! We're glad to invite you to take part in [contest:1909], which will start on [contest_time:1909]. You will be given **9 problems** and **3 hours** to solve them. One of the problems will be divided into two subtasks.
The problems were authored and prepared by me.
<spoiler summary="Spoiler">
</spoiler>
We would like to thank
- [user:errorgorn,2023-12-20] for his fun 🤌 🤡 🤓 😩 coordination;
- [user:Alexdat2000,2023-12-20] for Russian translation;
- [user:franv,2023-12-20] for writing $173$ lines of checker of problem H correctly at the first try;
- [user:franv,2023-12-20] for pictures in the editorial;
- [user:dario2994,2023-12-20], [user:Endagorion,2023-12-20], [user:ffao,2023-12-20], [user:Golovanov399,2023-12-20], [user:nor,2023-12-20] for VIP testing;
- [user:Kaey,202...
We hope you'll like the problemset!, We would like to thank
Announcement of Pinely Round 3 (Div. 1 + Div. 2)
100.
Polygon: AI-Powered Automatic Tips Hello, Codeforces!
<i>For the new members of the community, I would like to remind you that Polygon (https://polygon.codeforces.com/) is a system developed
and maintained by Codeforces for preparing programming problems. It is there that authors and coordinators develop all the problems for
the rounds. Moreover, I believe a significant (large?) portion of the problems for other competitions is also developed there: various
stages of ICPC, national competitions of different levels, educational problems for various courses, etc.
<b>In 2023, more than 50000 problems were prepared in Polygon (only those for which a package was compiled are counted)!</b></i>
Large language models are becoming an increasingly integral part of our lives, taking over routine and monotonous tasks.
With their help, problems created in Polygon will now have fewer typos and incorrect language constructions.
We have recently implemented three features in Polygon to ease routine tasks:
* Translati...
For the new members of the community, I would like to remind you that Polygon
(https, I would like to note that I see no reason to worry about data leakage when
processing statements
101.
Codeforces Round #292 Hello, everyone! Codeforces Round #292 will be held at [Feb/17/2015 19:30 MSK](http://timeanddate.com/worldclock/fixedtime.html?day=17&month=2&year=2015&hour=19&min=30&sec=0&p1=166). We're looking forward to your participation!
The problems are from [user:dreamoon,2015-02-16], and thanks [user:Shik,2015-02-16] for some discussion. Also we want to thank [user:Zlobober,2015-02-16] for helping me prepare the round, [user:AlexFetisov,2015-02-16] and [user:winger,2015-02-16] for testing this round , [user:delinur,2015-02-16] for translating the statement into Russian, and [user:MikeMirzayanov,2015-02-16] for Codeforces and Polygon.
This is first time I provide all problems for a Codeforces round. I hope you'll find it interesting! Please read all problem statements and discover what the main character [user:drazil,2015-02-16] do in those problems for he's really cute =)
Finally, I would like to ask [user:sorry_dreamoon,2015-02-16] to participate this round. I believe everyone have ...
Finally, I would like to ask [user:sorry_dreamoon,2015-02-16] to participate
this round. I believe
Announcement of Codeforces Round 292 (Div. 2)
Announcement of Codeforces Round 292 (Div. 1)
102.
Codeforces Round #767 (Div. 1, Div. 2) #### Hello Codeforces!
[user:magnus.hegdahl,2022-01-18] and I are glad to invite you to [contest:1628] and [contest:1629] which will be held on [contest_time:1629]! This round is rated for both divisions.
In each division there will be **6 problems** and **2 hours** to solve them.
We would like to thank the following amazing people:
- [user:74traktor,2022-01-18] for great coordination of the round and translating the statements!
- [user:efimovpaul,2022-01-18], [user:agrayush13,2022-01-18], [user:ak2006,2022-01-18], [user:BidoTeima,2022-01-18], [user:QuangBuiCP,2022-01-20], <a class="rated-user user-blue" href="/profile/mesanu" title="mihai">mesanu</a>, <a class="rated-user user-blue" href="/profile/_Vanilla_" title="chad">_Vanilla_</a>, [user:down,2022-01-18], [user:FedShat,2022-01-18], [user:gupta_samarth,2022-01-21], [user:namanbansal013,2022-01-21], [user:MoonKnight.,2022-01-18], [user:stefdasca,2022-01-18], [user:AlperenT,2022-01-18], [user:DivanCode,2022-01-20], [user...
We would like to thank the following amazing people:
Announcement of Codeforces Round 767 (Div. 1)
Announcement of Codeforces Round 767 (Div. 2)
103.
Stream I want to try streaming competitive programming. The goal is to make it educational so I will be talking a lot, also answering (at least some) questions.
I'm planning three completely different streams to see what format to use:
1. Solving random easy-medium problems.
2. Solving old problems from Polish olympiad.
3. Post-contest stream, assuming I'm a setter or tester of a contest.
Similar to what [user:scott_wu,2018-09-27], [user:ecnerwala,2018-09-27] and [user:stevenkplus,2018-09-27] did.
Any thoughts, ideas?
The first stream will most likely be on Sunday, starting around 7-8pm CEST.
I will use Twitch and talk in English only.
These first streams should show me and you whether it's cool and useful, and whether I like doing it.
Also, I will try to make the video accessible later.
**--- UPDATE ---**
The first stream will start on Saturday at 10am CEST (check your [timezone](https://www.timeanddate.com/worldclock/fixedtime.html?msg=Errichto+Stream+%231&iso=2018...
whether I like doing it.
104.
Codeforces: Results of 2019 <a style="float:right; display: inline-block; margin:0 1em 1em 2em; max-width: 60%;" href="https://codeforces.me/10years"/>
<img style="max-width:100%" src="https://assets.codeforces.com/images/codeforces-10-years-300.png"/>
</a>
Hi Codeforces!
The platform recently turned 10 years old. We accept your congratulations at <a href="https://codeforces.me/10years">https://codeforces.me/10years</a>.
Please review the progress in 2019 and support the anniversary crowdfunding campaign. Your donation will be a contribution to the future of Codeforces, will
help its development and work. We need resources to continue to host rounds, not to stop developing,
to innovate and to maintain the existing infrastructure.
In 2019, together with numerous writers the coordinators worked on problems: [user:KAN,2020-02-06], [user:300iq,2020-02-06], [user:cdkrot,2020-02-06], [user:arsijo,2020-02-06] and recently joined [user:isaf27,2020-02-06]. And this year,
rounds coordinated by other exper...
plus a third! How do you like that, Elon Musk?
105.
I am Mike Mirzayanov. AMA! <img src="/predownloaded/3f/31/3f31c3ac1f7b4e53128fc9708f5736eea40d7b94.jpg" style="width:250px; margin:1em 0 0 0; float:right;"/>
But why not!
Many people here seem to know me. Or maybe not quite? I am ready to answer your questions. Ask me anything! I plan to spend no more than 3 hours on answers. Sorry if I can't answer all the questions.
Let's more or less follow the rules from here https://www.reddit.com/r/IAmA/wiki/index
Below is actually a quote from the rules.
The interviewee begins the process by starting a post, describing who they are and what they do. Then commenters from across the internet leave questions and can vote on other questions according to which they would like to see answered.
The interviewee can go through and reply to the questions they find interesting and easily see those questions the internet is dying to have the answer to.
Because the internet is asking the questions, they're going to be a mix of serious and lighthearted, and you'll ...
which they would like to see answered.
106.
My thoughts on clarifications and on the round #677. Alright, I'm done. I wanted to write a blog like that for a long time, but now I really don't have enough patience to ignore this issue anymore.
I want to say that I get 100-200 clarifications each Div.3 round. It doesn't even matter how well the statements are, it doesn't matter if there are any issues with solutions or checkers, people always find what to ask.
Before I start whining here, I really want to recommend anyone to read these two [user:Um_nik,2020-10-20] blogs: [this one](https://codeforces.me/blog/entry/62730) and [this one](https://codeforces.me/blog/entry/62744). Sorry, [user:Um_nik,2020-10-20], I unnecessarily tagged you, I will not do that anymore.
Now some breaking news for most participants: _your local IDE compiler_ and _Codeforces compiler_ are **actually** different things! If your code works fine locally but gets WA/TL/RE/ML/etc on Codeforces, it (most probably) means you have a bug somewhere in your code. There are rare cases when the issue is with comp...
**during the time we were figuring out if we really have to make it unrated**.
I got somethinglike, got like $10-20$ questions like "can there be several answers???" but it was
bolded in the statement)., or something like that, I understand that I'm not perfect and make a lot of
mistakes., you think you have to do? Of course write a question about that! I got like 50
questionslike this, Alright, I'm done. I wanted to write a blog like that for a long time, but now
I really don't have, like $30$ seconds trying to understand that) and will wait for the real
question instead of the
107.
Codeforces Round #743 Hello Codeforces!
[user:Asymmetry,2021-09-16] and I are glad to invite you to [contest:1572] and [contest:1573], which will be held on [contest_time:1572].
Each division will have **6 problems** and **2 hours** to solve them. All problems were written and prepared by [user:Asymmetry,2021-09-16] and me. The round will be rated for both divisions.
We would like to thank:
- [user:1-gon,2021-09-16] for coordinating the round and helping us with the problems.
- [user:mnbvmar,2021-09-16], [user:gamegame,2021-09-16], [user:dorijanlendvaj,2021-09-16], [user:tnowak,2021-09-16], [user:Jasiekstrz,2021-09-16], [user:ijxjdjd,2021-09-16], [user:shxgun,2021-09-16], [user:lcaonmst,2021-09-16], [user:-_-INV4D3R-_-,2021-09-16], [user:ToxicPie9,2021-09-16], [user:Shinchan01,2021-09-16] and [user:Krzaq,2021-09-16] for testing the round and providing useful feedback.
- [user:ToxicPie9,2021-09-16] for being a VIP tester.
- finally, [user:MikeMirzayanov,2021-09-16] for great platforms Codeforc...
**UPD:** We would also like to thank [user:Ari,2021-09-16] for testing the
round and [user:KAN,2021, We would like to thank:
Announcement of Codeforces Round 743 (Div. 1)
Announcement of Codeforces Round 743 (Div. 2)
108.
Remove Virtual Contestants from Contest Leaderboards As most of you are likely aware, cheating in virtual contests (typically by submitting prewritten or copied solutions in order to appear at the top of the leaderboard) is very common. Most recently, [user:wannabecandidatemaster,2020-05-28] submitted solutions to all problems in [this round](https://codeforces.me/contest/1358/standings) in order to take first place in the standings, and [user:bemandrei,2020-05-28] competed virtually in [this round](https://codeforces.me/contest/1360/standings) after competing in the round officially, and ended up in second place on the leaderboard.
In response to the rise of VC cheating (as well as some other issues with the VC system I'll describe below), I'd like to make a simple proposal: **Remove all virtual participants from the contest leaderboards.** The one exception is that you should see your own name on leaderboards for contests you VCed (that is, virtual contestants should be visible only to themselves). Similarly, virtual participan...
describe below), I'd like to make a simple proposal: **Remove all virtual
participants from the contest, rankings for out of contest participants. Though this may seem like a
relatively minor issue, Div 2
109.
Codeforces Round #647 — Thanks, Algo Muse! Hello Codeforces!
We have a pleasure to invite you to [Codeforces Round #647 (Div. 1)](https://codeforces.me/contests/1361) and [Codeforces Round #647 (Div. 2)](https://codeforces.me/contests/1362). This round will take place on [contest_time:1361]. In both divisions, you will have **2.5 hours** to solve **6 problems**. Three of them will be shared.
The problems for this round were prepared by [user:MicGor,2020-06-02], [user:Grzmot,2020-06-02], [user:Okrut,2020-06-02] and me.
We would like to thank everyone who made this round possible:
- [user:Nebuchadnezzar,2020-06-02] for the excellent coordination;
- [user:Lewin,2020-06-02], [user:ALILILILILI-KHAN,2020-06-02] for invaluable advice and extensive testing;
- [user:KrK,2020-06-02], [user:yarek,2020-06-02], [user:-is-this-fft-,2020-06-02], [user:w0nsh,2020-06-02], [user:Marckess,2020-06-02], [user:Ari,2020-06-02], [user:Fortin,2020-06-02], [user:Dup4,2020-06-02], [user:kraborak,2020-06-02], [user:300iq,2020-06-02], ...
We would like to thank everyone who made this round possible:
Announcement of Codeforces Round 647 (Div. 1) - Thanks, Algo Muse!
Announcement of Codeforces Round 647 (Div. 2) - Thanks, Algo Muse!
110.
C++ tips and tricks Hello, codeforces.
I would like to tell you about some tricks and constructions I use in C++. They are not hidden or anything, moreover, most of them are from stl or maybe well known among software engineers, but I often see codes where something is done by hand in, say, 5 lines, while in stl there is a function which does the same.
The things below are filtered by my personal sense of non-popularity (so no `__builtin` functions) and usage frequency (so there almost surely are similar things I don't know about) and are not really sorted in any reasonable order. Let's begin. [cut]
<hr>
- ### `all(x)`
This may be an exception to the rule of non-popularity -- this is quite widely used, but some next items will depend on `all(x)`, so I define it here. So, I talk about
```[c++]
#define all(x) (x).begin(), (x).end()
```
Now sorting a vector looks like `sort(all(vec))` instead of `sort(vec.begin(), vec.end())`.
However, it's not all about this define. Imagine you nee...
(assume it's `rand()` calls) may look like, doesn't have a constructor of vector, you may go write a loop with an ok face,
like:, implement your own binary search like, implement your own merge sort). Some of you, probably, write some code like
this:, that some experienced guys do the subj like, that this is simply a cast to bool. The easiest bool casting operator is `!!`
and it workslike this:, useful name. To avoid this, one can wrap all this into a block like this:, variadic template in your, well, template, to call something like `min(a, b,
c, ...)`, but you may do this, $ elements from $n$. You may write something like this:, I bet many, _many_ of you at least once wrote something like, I would like to tell you about some tricks and constructions I use in C++. They
are not hidden or, I'm sure I forgot something. I also feel that this blog is like a quick start
guide into stl. Still, If a usual reading a graph looks like this:, If you want to print, say, last 20 bits of a number, it's not necessary to
write smthlike, Now sorting a vector looks like `sort(all(vec))` instead of `sort(vec.begin(),
vec.end())`., since it costs two calls of `f`. You may write something like this:
111.
Codeforces Round #652 (Div. 2) $$~-\text{In the name of God}~-$$
Hi community,
I'm glad to invite you to my first contest, [contest:1369] which will be held at [contest_time:1369] ($\text{notice the unusual time}$). The problems are mainly prepared and invented by me. The round is rated for participants with rating strictly less than $2100$, others are able to take part in the round out of competition. You will be given $2$ $\text{hours}$ to solve $6$ $\text{problems}$.
Firstly I'd like to thank [user:adedalic,2020-06-15] for coordinating and reviewing the round, as well as helping with many different things.
I'd like to thank [user:antontrygubO_o,2020-06-21], [user:physics0523,2020-06-21], [user:McDic,2020-06-21], [user:Ashishgup,2020-06-21], [user:dannyboy20031204,2020-06-21], [user:Kuzey,2020-06-21], [user:Return_0,2020-06-21], [user:FieryPhoenix,2020-06-21], [user:ma_da_fa_ka,2020-06-21], [user:ITDOI,2020-06-21], [user:AM_I_Learning,2020-06-21], [user:pikmike,2020-06-21], [user:lynmisakura,2020-06-...
given $2$ $\text{hours}$ to solve $6$ $\text{problems}$. Firstly I'd like to
thank [user:adedalic, Also I'd like to thank coauthors, [user:amiralisalimi,2020-06-21],
[user:AS.82,2020-06-21] and, Firstly I'd like to thank [user:adedalic,2020-06-15] for coordinating and
reviewing the round, as, I'd like to thank [user:antontrygubO_o,2020-06-21],
[user:physics0523,2020-06-21], [user:McDic,2020
Announcement of Codeforces Round 652 (Div. 2)
112.
Blowing up unordered_map, and how to stop getting hacked on it C++ has always had the convenient data structures `std::set` and `std::map`, which are tree data structures whose operations take $O(\log n)$ time. With C++11, we finally received a hash set and hash map in `std::unordered_set` and `std::unordered_map`. Unfortunately, I've seen a lot of people on Codeforces get hacked or fail system tests when using these. In this post I'll explain how it's possible to break these data structures and what you can do in order to continue using your favorite hash maps without worrying about being hacked
[cut]
.
So how are they hackable? We always assume hash maps are $O(1)$ per operation (insert, erase, access, etc.). But this depends on a key assumption, which is that each item _only runs into $O(1)$ collisions on average_. If our input data is completely random, this is a reasonable assumption. But this is no longer a safe bet when the input isn't random, especially so if someone is adversarially designing inputs to our code (a.k.a. hacking ph...
standard hash function looks something like this:, A slightly better hash function like the following may look enticing:, Personally, I like to use
[splitmix64](http://xoshiro.di.unimi.it/splitmix64.c), which is extremely
113.
[Tutorial] Matroid intersection in simple words **[This article is also available in [Russian](https://codeforces.me/blog/entry/69287?locale=ru)]**
Hello, CodeForces.
I think that matroids are beautiful and powerful concept, however, not really well known in competitive programming.
I’ve discovered matroids at 2019 Petrozavodsk Winter Training Camp. There was a problem that clearly cannot be solved using usual techniques I knew, editorial for this problem was just these three words “just matroid intersection”. Back then it took me more than 2 days of upsolving to find all the information and details I need and implement solution that gets Accepted on this. And it took way longer to actually understand why does it work and exactly how does it work. (I still hesitate in some details.)
Of course, it is not hard to google up all the definitions and some related articles, but in my opinion they all are focused more on mathematical part of theory, strict proofs in some not really obvious but short ways, and observing only ke...
that we need to do this for two matroids at the same time and also would like
to somehow obtain the, I am sure, there are a lot of people in CP who will also like to get familiar
with matroids. So, Surely, you can find all the information in google with queries like “matroid”,
“matroid theory, We can show multiple examples of such problems. I mostly like these:
114.
Hacking (as a CF feature) does not fit modern competitive programming 1. How does hacking work with the new feature of unrated registration? I have seen this question raised in the comments to the announcement several times, but there is no answer still and I think it's weird that Mike hasn't thought about it before announcing.
2. I was working recently on a div.2 Codeforces round (still might happen in the future) and I was told by my coordinator that pretests should be equal to systests in all the problems. From this I can conclude that it is a Codeforces policy and intentionally weak pretests do not happen. It doesn't mean that it is impossible to hack anybody, but it doesn't sound like a viable strategy. I find it weird that this information is not public, it's like I'm getting bonus information about all other Codeforces rounds by setting a round myself.
I have done exactly zero research on the matter, but my feeling is that 99% of hacks (I'm talking about during-the-round-hacking here, not open hacking of education rounds and such) happen in ...
anybody, but it doesn't sound like a viable strategy. I find it weird that
this information is not, mean that it is impossible to hack anybody, but it doesn't sound like a viable
strategy. I find it
115.
Codeforces Round 969 (Div. 1, Div. 2) It's been a long day without you, Codeforces!
We are glad to invite you to take part in [contest:2007] and [contest:2006], which will start on [contest_time:2007]. You will be given **6 problems** and **2 hours and 30 minutes** to solve them in both divisions.
The problems are authored by [user:LMydd0225,2024-08-27], [user:tzl_Dedicatus545,2024-08-27], [user:_FJqwq,2024-08-27], [user:Ternary_Tree_,2024-08-27] and me [user:wyrqwq,2024-08-27] and mostly prepared by [user:LMydd0225,2024-08-27]. One of the problems is also prepared by [user:DitaMirika,2024-08-27].
We would like to thank:
- [user:Artyom123,2024-08-27] for his intelligent coordination, and for Russian translation.
- [user:arvindf232,2024-08-27], [user:dXqwq,2024-08-27], [user:A_G,2024-08-27], [user:Atomic-Jellyfish,2024-08-27], [user:N_z__,2024-08-27], [user:Tony2_CF,2024-08-27], [user:abc864197532,2024-08-27], [user:PEIMUDA,2024-08-27], [user:hyman00,2024-08-27], [user:yuanruiqi,2024-08-27], [user:Imdie,2024-08...
We would like to thank:
Announcement of Codeforces Round 969 (Div. 1)
Announcement of Codeforces Round 969 (Div. 2)
116.
Codeforces Round #703 (Div. 2) Hi, Codeforces!
I finally get to return something to this great community, so I am glad to invite you to take part in [Codeforces Round #703 (Div. 2)](https://codeforces.me/contest/1486), which will take place on [contest_time:1486]. The round will be **rated for the participants with rating lower than 2100**. Participants from the first division are also welcomed to take part out of competition.
You will be given **2 hours 15 minutes** to solve **6 problems**. All the problems were created and prepared by me. One of the problems is interactive, please read [the guide of interactive problems](https://codeforces.me/blog/entry/45307) before the contest.
I would also like to thank:
<ul>
<li> [user:antontrygubO_o,2021-02-16] for excellent coordination! </li>
<li> [user:1-gon,2021-02-16], [user:3EVEHAR_KOAVA,2021-02-16], [user:Akulyat,2021-02-16], [user:Aleks5d,2021-02-16], [user:BRCode,2021-02-16], [user:Osama_Alkhodairy,2021-02-16], [user:PO3OBAR_Bblnb,2021-02-16], [user:...
I would also like to thank:
* [user:antontrygubO_o,2021-02-16] for excellent
Announcement of Codeforces Round 703 (Div. 2)
117.
Ozon Tech Challenge 2020 Hello Codeforces!
<center>
<img alt="ozon" src="https://codeforces.me/predownloaded/6a/99/6a99904b94e5c20a149198e2dde4068dadd9278a.png" style="float:right; height: 200px; margin: 10px; max-width:50%;"/>
</center>
We are glad to invite you to the upcoming “[Ozon Tech Challenge 2020](https://codeforces.me/contests/1305)”, which will start at [contest_time:1305]. The round will be open and rated for everybody!
This round is initiated and supported by Ozon.
Ozon — one of the leading players in e-commerce (provides its customers with over 2.5 million product names in 24 categories), a tech company that actively developing its IT department. They already have the largest Golang team in Russia, a proprietary WMS fulfillment management system completely written by the Ozon team, 250 million lines of logs per day, collected on the site and in the Ozon mobile application. Ozon experts perform at the leading profile conferences GoperCon, Heisenbug, GoWayFest, GolangConf, and also ...
We would like to thank:
118.
PinkieRabbit Cheated in Div. 1 Contest on Apr 27 I am writing to report an incident of cheating by PinkieRabbit during the recent contest 1965 on Codeforces. **PinkieRabbit asked help about problem D from others during the contest,** which is definitely against the contest rules.
I have attached the evidence of the cheating behavior in the appendix, please read it. This evidence clearly shows the communications PinkieRabbit had with another person during the contest. It is a contest recording video published after contest by himself. As a former fan of PinkieRabbit, after seeing this video, I was shocked by how dishonest PinkieRabbit was. I believe such actions undermine the integrity of the competition and the platform as a whole. [user:win114514,2024-05-05] cheated by grouping up in contest 1905, finally punished by skipping. Therefore, I respectfully request that Codeforces take appropriate action against PinkieRabbit to maintain the fairness of the contests on Codeforces.
He only cheated in one problem this round, so I sugg...
7, heck, how you ...... how it's just like that., F: How can you using the map like that?, [Voice of unknown person]:That's what it feels like.
119.
CodeTON Round 7 (Div. 1 + Div. 2, Rated, Prizes!) Hello Codeforces,
We are very glad to invite you to participate in [contest:1896], which will start on [contest_time:1896]. You will be given **8 problems** and **2.5 hours** to solve them. The round will be rated for everyone.
All the problems are written and prepared by [user:lanhf,2023-11-22], [user:Mike4235,2023-11-22], [user:thenymphsofdelphi,2023-11-22], [user:xuanquang1999,2023-11-22] and me.
We would like to give our sincere thanks to:
- [user:errorgorn,2023-11-22] for his wonderful coordination!
- [user:Alexdat2000,2023-11-22] for translating problem statements.
- [user:Um_nik,2023-11-22] and [user:conqueror_of_tourist,2023-11-22] for Legendary Grandmaster testing.
- [user:jeroenodb,2023-11-22], [user:Kuroni,2023-11-22], [user:dario2994,2023-11-22] and [user:dreamoon_love_AA,2023-11-22] for International Grandmaster testing.
- [user:generic_placeholder_name,2023-11-22], [user:MofK,2023-11-22], [user:minhcool,2023-11-22], [user:dvdg6566,2023-11-22] and [user:mag...
We would like to give our sincere thanks to:
Announcement of CodeTON Round 7 (Div. 1 + Div. 2, Rated, Prizes!)
120.
[Tutorial] Supporting Priority-Queue-like Undoing on DS _Good morning!_
In this blog, I will present an online algorithm that can perform priority-queue-like undoing on a data structure, given a few assumptions. I will also present and solve a generalization of that problem.
For context, I highly recommend reading this blog that shows how to solve the easier problem of queue-like undoing. That blog gave me the inspiration for the trick I will describe here, but I will still try to write this blog so that it can be understood without it. Thank you [user:Noam527,2023-01-08] for introducing such a great trick to the competitive programming community.
[[Tutorial] Supporting Queue-like Undoing on DS](https://codeforces.me/blog/entry/83467)
Acknowledgment
-----------------
Huge thanks go to [user:peltorator,2023-02-20] for hosting a [blog contest](https://codeforces.me/blog/entry/110840). His generous prize of $300 got many people to write blogs on new, interesting ideas, and it was one of the main reasons I wrote this blog. I'm...
[Tutorial] Supporting Priority-Queue-like Undoing on DS, queue-like undoing. That blog gave me the inspiration for the trick I will
describe here, but I will, Although priority-queue-like undoing is pretty cool, I realized it's still not
general enough to, First, if the priorities are inserted in decreasing order, this is the same as
the queue-like, In this blog, I will present an online algorithm that can perform
priority-queue-like undoing on a, Note that this can simulate deque-like undoing because we can use priorities
$(p, -p)$., Now, we would like to the solve the problem of supporting the following types
of operations online:, [[Tutorial] Supporting Queue-like Undoing on
DS](https://codeforces.me/blog/entry/83467)
121.
Good Bye 2017 Hello everyone!
Another year has gone by. The last Codeforces contest of this year will be tomorrow, [29 Dec, 15:35 UTC](https://www.timeanddate.com/worldclock/fixedtime.html?day=29&month=12&year=2017&hour=18&min=35&sec=0&p1=166).
The round details:
- combined div1+div2
- 8 problems
- 2.5 hours
- rated!!!
I'd like to thank the following people for helping with the round: [user:KAN,2017-12-28], [user:winger,2017-12-28], [user:AlexFetisov,2017-12-28], [user:zemen,2017-12-28], [user:xiaowuc1,2017-12-28], [user:MikeMirzayanov,2017-12-28]. Without them, this round would not have been possible.
Scoring distribution will be posted later. Make sure to read ahead on the problems, since there may be some later problems that are easier for you. I hope to see you all at the contest, and good luck on the last chance to increase your rating this year!
**EDIT1:** The scoring distribution is **500-750-1000-1750-1750-2000-2750-3500**
**EDIT2:** There will be a five minute delay ...
I'd like to thank the following people for helping with the round:
[user:KAN,2017-12-28
122.
Evidence of ACPC Corruption This blog is about ACPC corruption, ACPC is the ICPC regional contest for students competing in Africa and the Arab region.
Sorry for posting this from a fake account, but as you will see in the video our team can be disqualified in case we used our real accounts!
We believe that ACPC is corrupted and in this video, we have included some evidence of this corruption
<iframe width="560" height="315" src="https://www.youtube.com/embed/sPZGx9uXEC8" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
FB Page: https://www.facebook.com/ACPC-Corruption-104279088364028
We are students, we can’t afford to pay **$1500** (the mandatory participation fees). We don’t understand why we are the only region where students have to pay these huge sums. All we want is a normal contest that students can afford its fees like any other region.
ACPC has set a deadline for paying the fees on **15/02** and ...
normal contest that students can afford its fees like any other region.
123.
Codeforces Round #932 (Div. 2) <img alt=" " src="https://i.imgur.com/EFlomVE.png" style="float: right;max-width: 100.0%;max-height: 100.0%;width:30%;height:30%">
<h2>Hello, Codeforces! 😇</h2>
------------------
We are glad to invite you to participate in [contest:1935], which will take place at [contest_time:1935] 🔥 😱 🤩
The round has been fully prepared by students of the School CPM 💓 (School of the Pedagogical Skills Center): [user:IzhtskiyTimofey,2024-02-25], [user:AndreyPavlov,2024-02-25], and myself. The theme of all the problems will be our beloved school 😈
This round will be **rated** for participants with a rating below **2100** 😊 Participants with a higher rating can take part out of competition 😋
We would like to thank 💕:
- Our wonderful coordinator [user:Artyom123,2024-02-25] for helping prepare the problems and valuable advice! 😍
- CPM students [user:azureglow,2024-02-25] and [user:max0000561,2024-02-25] ...
We wish everyone good luck in the round, and we would also like to wish good
luck to the, We would like to thank 💕:
Announcement of Codeforces Round 932 (Div. 2)
124.
Blogewoosh #5 (with images from now) Hello, codeforces!
Because after Round #507 sad men in suits visited me in my flat, this time I won't write about any task from the future. Instead, this blog will be about my own trick. I said "my own," but probably some of you have already heard about it or even figured it out, but I've developed it by myself, so I consider it as my own.
In particular, it's **GEOMETRY TIME!!!**. But please, don't escape already. I also don't like this topic so much, that's why I really like this trick. Let me tell you a story from one onsite Polish contest, which took place a few months ago. I was thinking about one of the problems, and I've figured out that I had to do some binary-search (on doubles) and then check if a set of half-planes has a non-empty intersection. The answer would tell me in which direction should I turn in the binary search.
Firstly, I grabbed my head, because I've never written an intersection of half-planes. I had my acm library with [user:Errichto,2018-09-07]'s code...
"). Don't calculate it, just focus on the fact that it exists. It looks like
this:, In particular, it's **GEOMETRY TIME!!!**. But please, don't escape already. I
also don'tlike this
125.
think-cell Round 1 Hello, Codeforces!
Welcome to the [contest:1930] supported by think-cell, which will start on [contest_time:1930]. It will be a combined rated round for both divisions.
All problems were authored and prepared by [user:Elegia,2024-02-13] and [user:satyam343,2024-02-07].
We would like to thank:
- [user:errorgorn,2024-02-13] for round coordination and help with preparation
- [user:KAN,2024-02-17] for helping with the preparation of problems
- [user:Alexdat2000,2024-02-13] for Russian translation
- [user:GoatTamer,2024-02-17] and [user:Non-origination,2024-02-17] for discussing problems with us while our proposal was in review and testing the round.
- [user:Kaitokid,2024-02-13], [user:antekb,2024-02-13], [user:vgtcross,2024-02-13], [user:riano_,2024-02-13], [user:mtw,2024-02-13], [user:AboAbdoMC,2024-02-13], [user:Vladithur,2024-02-13], [user:BERNARD,2024-02-13], [user:milind0110,2024-02-13], [user:conqueror_of_tourist,2024-02-13], [user:Um_nik,2024-02-13], [user:gamegame...
We hope you'll like the problemset!, We would like to thank:
Announcement of think-cell Round 1
126.
C++20 Is Released Hello, Codeforces.
Please, welcome c++20 support on Codeforces. Yes, it is 64-bit. Thanks to Brecht Sanders: I used his distribution GCC-11.2.0-64 from https://winlibs.com/.
If you have installed [PBOX](http://pbox.me/packages), you can add this compiler with the line `pbox install gcc11-64-winlibs`. Probably, a good idea is to add `C:\Programs\gcc11-64-winlibs\bin` into the PATH. More about PBOX you can read [here](https://codeforces.me/blog/entry/18274).
I use the compilation command line similar to other GCC installations: `g++ -Wall -Wextra -Wconversion -static -DONLINE_JUDGE -Wl,--stack=268435456 -O2 -std=c++20 <source>`. The only differences are `-std=c++20` and `-Wall -Wextra -Wconversion` (I plan to use somehow such warnings in Polygon to suggest fixes in uploaded files).
Now you can use c++20 in your solutions. I'm not sure there are many features useful in competitive programming. Probably, I'm wrong. For example, now you can write `vector v{vector{1, 2}};` inste...
happens that some specifiers like `lld` or `Lf` work unexpectedly. In the
table by reference, the
127.
Codeforces Round #741. How contest author is entertaining. Hello, Codeforces! Not too long ago there was my second round, Codeforces #741 (Div. 2). Before it started, I wondered: surely there will be a lot of cheating on my round, just like on any other? So I decided to do something unusual.
As you know from [a blog about cheaters] (https://codeforces.me/blog/entry/93893), it's easy to find telegram/youtube channels with cheaters. So I decided to find a couple and join them...
So, the first telegram group I found was [link not available yet]. There weren't too many cheaters in it but they were, here's an example:
<spoiler summary="Picture">
<a href="https://imgbb.com/"><img src="https://i.ibb.co/kJqMzMS/pic1.png" alt="pic1" border="0"></a>
</spoiler>
I joined this telegram group, and there was a meaningful dialogue:
<spoiler summary="Picture">
<a href="https://ibb.co/1mC2csG"><img src="https://i.ibb.co/VqFB6HY/pic2.png" alt="pic2" border="0"></a>
</spoiler>
This may seem unbelievable, but the admin of the group actuall...
direct insults, but to summarize, there were messages like this:, started, I wondered: surely there will be a lot of cheating on my round, just
like on any other? So
128.
Likes in Posts and Comments Hello.
Here's a small improvement for posts and comments. Likes can now be specified in their markup. Maybe this will be useful for organizing some kind of polls or collecting feedback.
Here are some examples:
Markup <br><small>Use square brackets instead of round</small> | Result
- | -
<small>Simple likes widget</small><br/>`(likes:1)` |
<small>Same keys means same likes widgets</small><br/>`(likes:2)`<br/>`(likes:2)` |
<br/>
<small>Same keys and different groups can be used for exclusive choices</small><br/>`(likes:3,option1)`<br/>`(likes:3,option2)` |
<br/>
Yes, I know that this feature can be improved. I would prefer that we try to use it a little and understand what exactly is missing the most. Maybe we need to implement the fully functional polls.
Let's do a small survey using the new feature.
1. What is your age range?
* under 16:
* 16-20:
* 21-25: ...
129.
Editorial for Hello 2024 ### [problem:1919a]
Author: [user:maomao90,2024-01-02]
<spoiler summary="Hint 1">
When does the game end?
</spoiler>
<spoiler summary="Solution">
Depending on whether the player chooses to exchange wallets with their opponent on step $1$, $1$ coins will be removed from either the opponent's wallet or the player's wallet. This means that if either of the players still has remaining coins, the game will not end as at least one of the choices will still be valid.
The only way that the game ends is when both players have $0$ coins. Since each operation decreases the total amount of coins by exactly $1$, the only way for Alice to win the game is if $a + b$ is odd.
</spoiler>
<spoiler summary="Code">
~~~~~
#include <bits/stdc++.h>
using namespace std;
int main() {
int t; cin >> t;
while (t--) {
int a, b; cin >> a >> b;
if ((a + b) % 2 == 0) {
cout << "Bob\n";
} else {
cout << "Alice\n";
}
}
...
ReLU-like functions together. More precisely, ReLU segment tree can solve the
following problem:, Unfortunately, it seems like
[ARC146E](https://atcoder.jp/contests/arc146/tasks/arc146_e) is, like?
Tutorial of Hello 2024
130.
touristreams are back! 006: SNSS 2020 R1, and more It's this time of the year again!
Tune in at [twitch.tv/the__tourist](https://twitch.tv/the__tourist) at around [21:45 MSK, 21 Aug 2020](https://www.timeanddate.com/worldclock/fixedtime.html?msg=touristream+006&iso=20200821T2145&p1=3396) to watch me solving [SnarkNews Summer Series 2020 Round 1](https://contest.yandex.com/snss2020/contest/19320/enter) live! First I will do the round competitively without much interaction, and I'll be up for discussing the problems on stream afterwards.
SnarkNews Winter/Summer Series (SNWS/SNSS) is organized by [user:snarknews,2020-08-19] yearly and consists of 5 rounds. Each round is a virtual 6-problem 80-minute contest open for 10 days. The series is open to all, but problem statements are in Russian only. Unlike last series, I'll be reading problem statements automatically translated to English on stream.
To make the stream more fun and for easier understanding of what will be going on, you're welcome to do the round yourself beforehand! If...
[contest link](https://contest.yandex.com/snss2020/contest/19321/enter/) if
you'dlike to do the contest or
131.
Codeforces Global Round 21 Hello Codeforces!
On [contest_time:1696] we will hold [contest:1696]. **<img src="/predownloaded/2e/e2/2ee2f0f95f05aae6ca6f47812dae2dc2aabf277b.png" style="width: 200.0px;float: right;margin: 0 1.0em 1.0em 1.0em;max-width: 100.0%;max-height: 100.0%;">**
It is the third round of a 2022 series of [Codeforces Global Rounds](https://codeforces.me/blog/entry/65002). The rounds are open and rated for everybody.
The prizes for this round:
- 30 best participants get a t-shirt.
- 20 t-shirts are randomly distributed among those with ranks between 31 and 500, inclusive.
The prizes for the 6-round series in 2022:
- In each round top-100 participants get points according to the [table](https://pastebin.com/QT5sXEaT).
- The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
- The best 20 participants over all series get sweatshirts and place certificates.
Thanks to XTX, which in 2022 supported the global ...
We would also like to thank the following people:
Announcement of Codeforces Global Round 21
132.
Codeforces Round #641 Hello, Codeforces!
We are excited to invite you to [contest:1349] and [contest:1350]. This round will take place on [contest_time:1349]. In both divisions, you will have **2.5 hours** to solve **6 problems**. **Please notice the unusual time.**
Problems of this round were prepared by [user:Fulisike,2020-05-04], [user:A.K.E.E.,2020-05-04], [user:mydiplomacy,2020-05-04] and me [user:ProgSlacking,2020-05-04].
We would like to express our sincere gratitude to:
- [user:300iq,2020-05-04], for responsible and interesting coordination;
- [user:WZYYN,2020-05-04], [user:Elegia,2020-05-04], [user:skip2004,2020-05-11], [user:vintage_Vlad_Makeev,2020-05-11], [user:fpdqwq,2020-05-04], [user:wangziji,2020-05-04], [user:Suika_predator,2020-05-04], [user:wrg0ababd,2020-05-04], [user:AcF-_-FcA,2020-05-04], [user:Kirill_from_Kazan,2020-05-04], [user:dysyn1314,2020-05-04], [user:xiaolou0411,2020-05-04], [user:LiM_256,2020-05-04], [user:HazyKnight,2020-05-04], [user:Sad_reacts_only,2020-05-0...
We would like to express our sincere gratitude to:
Announcement of Codeforces Round 641 (Div. 1)
Announcement of Codeforces Round 641 (Div. 2)
133.
Codeforces Round #670 (Div. 2) Hello Codeforces!
[user:gyh20,2020-09-10] and I are glad to invite you to [contest:1406] which will start on [contest_time:1406]. **Note the unusual start time of the round.**
The contest will last for **two hours**, and you will have **five tasks** to solve. The tasks are prepared by me and [user:gyh20,2020-09-10]. This round is rated for participants whose **rating is not higher than 2099**. You can see that my current rating is exactly **2099** :)
There might be an interactive problem. You can learn about them [here](https://codeforces.ml/blog/entry/45307).
We would like to thank:
- [user:isaf27,2020-09-10] for coordinating and helping us with the problems.
- [user:namanbansal013,2020-09-10], [user:postscript,2020-09-10], [user:morzer,2020-09-10], [user:Osama_Alkhodairy,2020-09-10], [user:csani,2020-09-10], [user:growup974,2020-09-10], [user:physics0523,2020-09-10], [user:bizhouziwei20,2020-09-10] for testing the problems and providing useful feedback. Special thank...
. We hope you like the problems!, We would like to thank:
Announcement of Codeforces Round 670 (Div. 2)
134.
Codeforces Global Round 15 <img src="/predownloaded/2e/e2/2ee2f0f95f05aae6ca6f47812dae2dc2aabf277b.png" style="width:200px; float:right; margin: 0 1em 1em 1em"/>
Hi!
On [contest_time:1552] we will host [contest:1552].
This is the third round of the 2021 series of [Codeforces Global Rounds](https://codeforces.me/blog/entry/65002). The rounds are open and rated for everybody.
The prizes for this round are as follows:
- 30 best participants get a t-shirt.
- 20 t-shirts are randomly distributed among those with ranks between 31 and 500, inclusive.
The prizes for the 6-round series in 2021:
- In each round top-100 participants get points according to the [table](https://pastebin.com/QT5sXEaT).
- The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
- The best 20 participants over all series get sweatshirts and place certificates.
Thanks to XTX, which in 2021 supported the global rounds initiative!
Problems for this round ar...
Announcement of Codeforces Global Round 15
135.
A summary of what passes for an Indian ICPC Contest, apparently The online round for the India’s ICPC was held on 23rd. And also a week before that. The first time around, the organisers tried debuting a self-hosted DOMjudge instance. That didn’t go too well, so a recontest was scheduled on CodeChef.
If you’re not too familiar with the Indian ICPC system — this online prelims is used to select teams that can participate in the offline regionals. From a single university, usually approx. 2 teams (for the top unis) or 1 (for almost all unis that aren’t in top-15) qualify.
As you could guess, it’s quite easy for good teams to miss a spot. Looking at the first contest’s rankings, teams from rank 2 to 100+ all had 4 solves. Owing to severe system issues in the first contest, the time penalties weren’t too reliable differentiators, leading to the recontest this Saturday.
In hindsight, we forgot to be grateful for the problemset and reliable testcases the first time. I’m sure everyone agrees that the basic requirements (ignoring all other stuff...
. I’m sure everyone agrees that the basic requirements (ignoring all other stuff
like problem quality
136.
Spectral::Cup 2026 Round 2 (Codeforces Round 1100, Div. 1 + Div. 2) <center>
<img src="https://i.imgur.com/lPGWfOR.png" style="width: 300.0px;max-width: 100.0%;max-height: 100.0%;"></center>
<center>
<h1>
Greetings Codeforces!
</h1>
</center>
[user:sammyuri,2026-05-18], [user:myst-6,2026-05-18] and I are absolutely chuffed to bits to invite you all to have a go at Codeforces Round ${12}_{10}$ ([contest:2229]), kicking off at [contest_time:2229].
There will be **9** absolute cinema problems for you to solve over the course of **3 hours**, some problems will have multiple parts. This round will be combined for Division 1 and Division 2 and will be rated for everyone.
I’d like to raise a glass to the following brilliant minds for making this whole thing possible:
- [user:dominater069,2026-05-18] for splendid coordination.
- [user:um_nik,2026-05-18] for preliminary review.
- The fine assembly of testers: [user:arvindf232,2026-05-18], [user:evenimage,2026-05-18], [user:geothermal,2026-05-18], [user:sana,2026-05-18], [user:wansur,20...
I’d like to raise a glass to the following brilliant minds for making this
whole thing possible:
137.
EPIC Institute of Technology Round Summer 2025 (Codeforces Round 1036, Div. 1 + Div. 2) # Hello Codeforces Once Again
<img alt="" src="https://codeforces.me/predownloaded/4e/71/4e713a365fc86888c808b3b56d6fecfb529973e6.png" style="float:right; height: 154px; padding-left: 35px; padding-right: 73px; padding-top: 41px; padding-bottom: 33px;"/>
After months of hard work, [user:cry,2025-07-01], [user:Lilypad,2025-07-01] and I are extremely proud to welcome you to participate in [contest:2124] at [contest_time:2124]. This round is combined for Division 1 and Division 2, and it will be rated for everyone.
You will be given $3$ hours to solve $9$ problems. One problem will be split into two subtasks.
We would like to thank the following people for making the contest possible:
- [user:satyam343,2025-07-01] for outstanding coordination, working tirelessly with us to improve problem quality and donating his own problems to us. I cannot thank him enough.
- [user:Error_Yuan,2025-07-01] for proposing some ideas that weren't used in the end.
- [user:Benq,2025-07-01...
We would like to thank the following people for making the contest possible:
138.
On problemsetting 2 [Previous part](https://codeforces.me/blog/entry/70178)
Hi there again! Previous part with opinions of some amazing problemsetters turned out to be interesting for you, so I decided to gather some more opinions :D. I really hope that this post will help someone in their problemsetting future!
#### When and how did you start problemsetting?
[user:300iq,2020-01-11]: April, 2017
[user:maroonrk,2020-01-11]: In 2015 Summer, I hold the first contest with friends at high school club. It was a local Japanese contest.
[user:Radewoosh,2020-01-11]: In high school, so probably for 5 years. I was creating problems for other people from my school as my teacher told me that it's a completely different point of view.
[user:voidmax,2020-01-11]: It was 3 years ago. I was on plane and I didn't have a clue what to do in my free time. Then I realized that I can try myself in problemsetting. Later I created first and one of my favorite problem ([problem:963B]). I was full of inspiration a...
), [user:rng_58,2020-01-12] (I really like CDE from new AtCoder WTF, but I don't
like a lot of other, Also, I like mixing known things, the result can be surprising.
([problem:102341L]), I like this problem because it is one of the few constructive problems that I
created, and Ilike, I like this problem, because new ideas behind it may be used for a lot of other
problems!, [user:maroonrk,2020-01-11]: I often feel like that. At such time I stop
creating problems, and
139.
[Tutorial] Supporting Queue-like Undoing on DS In many data structures, the operation of "undo" on the last update can be implemented easily: we can (usually) maintain a stack of the updates, where each update on the stack holds the memory cells it changed, and their original values. To undo an operation, just revert all changes from the top update on the stack. To maintain good complexity, we require the updates to operate in non-amortized time. I've seen this being used multiple times on DSU (without path compression).
If we imagine the updates as a sequence, then we can push an update to the end, and pop an update from the end by undo. Then, this sequence is a stack of updates. Here we discuss the idea of having a queue of updates: we can add a new update, or undo the oldest update still active. Specifically, this blogpost attempts to solve the following problem:
Given a data structure that can support some updates, some queries and an "undo" operation (each with their own time complexity), how can we create a data structu...
[Tutorial] Supporting Queue-like Undoing on DS, (like some alternating case)., queries, but its undo operation works like a queue (and we don't support a
stack-like undo in the
140.
Petition to revoke unrated account permissions to post comments/blogs Unrated Account Disaster
==================
Recently there have been an obscene number of unrated accounts showing up. Several of them have inappropriate names, so I won't give examples because I want to keep this post clean, but I'm sure you have seen them in the comments. They are likely all by the same individual, possibly with a few copy-cat troublemakers as well.
Recently, certain ones have begun to spam useless comments on old posts to clog up the recent actions, effectively hiding all relevant conversations except on the blog posts they spam on. For whatever reason, these are overwhelmingly on [user:Errichto,2020-07-10]'s old blog posts. Occasionally there are also a flood of obscene blog posts made by the user themselves.
Here's an example of what I mean:
#### This is obnoxious and I would like to see it stop.
The current solution clearly doesn't work. As soon as they get banned or muted or whatever, they c...
#### This is obnoxious and I would like to see it stop.
141.
Codeforces Round 892 (Div. 2) #### <b><font size="6">Hello, Codeforces.</font></b>
<img src="/predownloaded/14/cc/14cc4faf55cf00829bc2720470af02030a8165c2.jpg">
We are happy to invite you to [contest:1859], which will take place on [contest_time:1859]. All of the problems are original and were prepared by a collective of authors, consisting of [user:kristevalex,2023-08-09], [user:i_love_penguins,2023-08-09], [user:efimovpaul,2023-08-09] and me, [user:induk_v_tsiane,2023-08-09]. <b> This round will be rated for participants with a rating lower than 2100.</b>
We would like to thank:
- [user:Artyom123,2023-08-09] for coordinating the round.
- [user:maomao90,2023-08-09], [user:Yuu,2023-08-09], [user:Qwerty1232,2023-08-09] and [user:Mangooste,2023-08-09] for GM testing.
- [user:maks_matiupatenko,2023-08-09], [user:tiom4eg,2023-08-09], [user:ilyakrasnovv,2023-08-09], [user:sergeev.PRO,2023-08-09], [user:EJIC_B_KEDAX,2023-08-09], [user:Yar_sem,2023-08-09], [user:gmusya,2023-08-09], [user:zwezdinv,2023-08...
We would like to thank:
Announcement of Codeforces Round 892 (Div. 2)
142.
Codeforces Round 1024 (Div. 1, Div. 2) Hello, Codeforces!
[user:eren__,2025-05-05], [user:sweetweasel,2025-05-05], and I are glad to invite you to [contest:2101] and [contest:2102]. Both rounds take place at [contest_time:2101], and have a duration of **2 hours** and **30 minutes**.
The Division 1 contest is made up of $6$ problems, and the Division 2 contest is also made up of $6$!
We would like to thank:
- [user:maomao90,2025-05-05] for his wonderful coordination and *huge* help in preparing the round.
- [user:Alexdat2000,2025-05-08] for the Russian translations.
- Our testers: [user:mmdrzada,2025-05-05], [user:N_z__,2025-05-05], [user:Hamed_Ghaffari,2025-05-05], [user:Shahrad_Mirzaei,2025-05-05], [user:RSAMSD,2025-05-05], [user:-this-is-obd-,2025-05-05], [user:AmShZ,2025-05-05], [user:AlefHeKaaf,2025-05-05], [user:beny_kh,2025-05-05], [user:Nariman_MT,2025-05-05], [user:sohsoh,2025-05-05], [user:Shayan86,2025-05-05], [user:Nimazare,2025-05-05], [user:Amoo_Safar,2025-05-05], [user:_R00T,2025-05-05], [user:Um...
We would like to thank:
Announcement of Codeforces Round 1024 (Div. 1)
Announcement of Codeforces Round 1024 (Div. 2)
143.
Confession: I sent problem H to goodbye Going to be 100% honest here, I sent problem H to goodbye. I feel really bad to authors of Goodbye 2023 for ruining their round, it seems the round would have been better by just removing H. As much as the community is out here roasting [user:74TrAkToR,2023-12-31], I also had a part to play in this contest. I wish to be as transparent as I can here without saying any classified information (apologies to [user:KAN,2023-12-31] and [user:74TrAkToR,2023-12-31] if anything here shouldn't be disclosed).
To give some context, here is what went down in goodbye round from my perspective:
- 13 days before contest: KAN asks me if I can source a 1E for goodbye. I asked traktor to send me the testing gym so that I could gauge how hard the 1E should be. He told me he will send me after he translates the statements. (UPD: Note that the IMO 1986 problem was already removed before this stage which is why a new 1E was needed, I never saw this IMO problem)
- 11 days before contest: traktor send it...
As much as I would like to think that I'm a "good" coordinator, stuff like me
putting problems that
144.
Codeforces: Results of 2020 [list some changes and improvements] Hello, Codeforces!
I understand that 2021 has been going on for a long time, but here I have picked up a subset of some of the improvements that the Codeforces team made in 2020. Soon I will publish numbers (no, charts) with statistics for 2020. In the meantime, I bring to your attention a list of changes and improvements.
By the way, this is a decent list. This is about half to a quarter of all changes. It's just that other changes are more often somewhere in the internals of the system and are not visible to users. Please read this list. Each item is the effort of someone from the team. Thanks to [user:geranazavr555,2021-03-14], [user:kuviman,2021-03-14] and [user:cannor147,2021-03-14] for their efforts. You've made our platform better! Well, by the way, I don't quit programming and many improvements were made by me.
The items on the list are written in a concise and informal form, many of the items I just copied from commit messages from git. If you want more details &mdash...
to a private contest * Support of announcements (broadcasts) in micro-websites
(like m1
145.
Good Bye 2025 Editorial We'd like to thank you all for participating in the contest, and hope you enjoyed it. Any feedback would be appreciated!
<!-- You lost the game. -->
<spoiler summary="Rate the contest">
- Great contest:
- Good contest:
- Average contest:
- Bad contest:
</spoiler>
---
<spoiler summary="Did Franklin (from problem D) succeed in massacring a village of innocent elves like Frieren and Dilhan (from problem E)?">
The official lore is that he got lost in a forest of oddly colored Christmas trees (from problem F) and decided instead to write this editorial.
</spoiler>
<!-- ########### -->
<!-- #PROBLEM A# -->
<!-- ########### -->
<h2>[problem:2178A]</h2>
<spoiler summary="Did you like the problem?">
- Great problem:
- Good problem:
- Average problem:
- Bad problem:
- Did not solve:
...
Tutorial of Good Bye 2025
146.
I've completed Project Euler It was a beautiful journey full of internet digging, searching for patterns, learning stuff, fighting with formulas on paper, making crazy observations, coming up with brilliant ideas, implementing crazy optimizations, waiting for the programs to finish, suffering when something was wrong, and so on (and each of the mentioned not once not twice took multiple hours).
It's been a couple of months since I was left with the last unsolved problem and finally I did it! I didn't give up and I obtained the answer alone, without anyone's help, like in the rest of the problems (I was using only internet sources created before the publication of the problem). I'm writing this blog because I am bursting with joy and I wanted to share it with the community. I highly recommend PE as most of the problems were definitely very high quality (and some were a real pain in the ass, but they still teach how to overcome stuff that you're uncomfortable with).
Here's a little souvenir for me:
![ ](/pr...
! I didn't give up and I obtained the answer alone, without anyone's help, like
in the rest of the
147.
Editorial of Global Round 11 #### General comments
Broadly speaking, problems A-B-C-D were "div2 problems", while F-G-H were "strong grandmaster problems" (with E staying in the middle). I did not expect anyone to solve all the problems and thus I decided to give the scoring F+G=H (so that maybe someone would have solved H).
<br><br>
Many of the problems (A, C, D, E, G) admit multiple solutions. Sometimes the core of the solution is the same (C, D) and sometimes the solutions are truly different (A, E, G).
<br><br>
If you are an experienced participant, I would like to hear your opinion on the problems. Feel free to comment on this post or send me a private message.
<br><br>
<spoiler summary="Overview of the problemset" >
The easiest problem of the contest, *A-Avoiding Zero*, is about rearranging an array of numbers. It is intended as a very easy problem that still requires to think.
Then, in *B-Chess Cheater* an intuitive (but nontrivial to prove) greedy approach is the way to go.
*C-The Hard Work of P...
If you are an experienced participant, I would like to hear your opinion on the
problems. Feel free
Tutorial of Codeforces Global Round 11
148.
How to ask for help in PM There was a blog today with a similar title, and although it was (I think, not really sure as I didn't get a good look) some kind of satire/joke blog, I wanted to post some serious advice about things I (don't) like to see when people are PMing me. However I decided that this wouldn't be in the spirit of the blog and decided to make a separate blog. And now that blog is deleted anyway.
While it's not a bad thing to write messages to more experienced users, most people who write me do things that make them very difficult to deal with. These are my opinions, but somewhy I feel that many reds will agree. Some things here feel silly to write because they are so obvious. But I'm only writing them because people regularly mess them up.
[cut]
$~$
####Use punctuation and spelling.
There are some [very simple rules](https://codeforces.me/blog/entry/82186#comment-689836) that make your messages infinitely more readable. I find it especially strange when many of these messagers call...
call me "sir", which is supposed to show respect, but then write like this. Is
this how you write to
149.
Codeforces Round #777 (Div. 2) Привки, Кодефорсес (ﾉ◕ヮ◕)ﾉ*:･ﾟ✧
------------------
_(Hello, Codeforces on Russian)_
<img src="/predownloaded/b1/4a/b14a9b0bf490c91e37290a3dca53f53b9f6680b0.jpg" style="float: right;margin: 0 1.0em 1.0em 1.0em;max-width: 100.0%;max-height: 100.0%;">
[user:Igorbunov,2022-03-10] and **I** are happy to invite you to participate in [contest:1647], all the tasks of which will be about a little girl Madoka. It will take place on [contest_time:1647]. The round will be **rated** for all participants with rating strictly lower than **2100**. You will have **2 hours** to solve **6 problems**.
I would also like to express my gratitude to the following people:
- To our cute coordinator [user:Artyom123,2022-03-10].
- To cute testers for cute feedback on cute tasks: [user:FelixDzerzhinsky,2020-09-14], [user:lesssia,2022-03-09], [user:eyakm1,2022-03-09], [user:_Vanilla_,2022-03-09], [user:morzer,2022-03-09], [user: welleyth,2022-03-09], [user:TeaTime,2022-03-09], [user:Dart-Xeyter,2022...
I would also like to express my gratitude to the following people:
Announcement of Codeforces Round 777 (Div. 2)
150.
Squarepoint Challenge (Codeforces Round 1055, Div. 1 + Div. 2) Annyeonghaseyo, Codeforces!
We invite you to participate in [contest:2152], which will be held on [contest_time:2152].
The round will be **rated for everyone**. You will be given **8 problems**, one of which is divided into two subtasks, and **3 hours** to solve them. One of the problems will be interactive, so make sure to read the [guide for interactive problems](https://codeforces.me/blog/entry/45307) before the contest.
The problems are authored and prepared by [user:as_dfsdf,2025-09-29], [user:djm03178,2025-09-29], [user:ko_osaga,2025-09-29], and [user:qwerasdfzxcl,2025-09-29].
We would like to thank:
- [user:errorgorn,2025-09-29] for great coordination.
- [user:Alexdat2000,2025-09-29] for great Russian translation.
- [user:Adam_GS,2025-09-29], [user:ainta,2025-09-29], [user:bangan,2025-09-29], [user:BuzzyBeez,2025-09-29], [user:cookiedoth,2025-09-29], [user:Dragos,2025-09-29], [user:gamegame,2025-09-29], [user:garam1732,2025-09-29], [user:Geothermal,2025-09-29],...
We would like to thank:
Announcement of Squarepoint Challenge (Codeforces Round 1055, Div. 1 + Div. 2)
151.
Petition to Ban FerjaniSassi for Mass Harassment and Spam Blog Entries Hello, CodeForces.
I'm writing this blog entry to petition to ban [user:FerjaniSassi,2024-06-28] for their mass harassment and spam blogs.
I was pinged by them for 4 times in their spam blogs, some of which were removed (probably by CodeForces).
After I checked their [blog](https://codeforces.me/blog/FerjaniSassi), I was shocked that they pinged **thousands** of users rated from Expert to LGM in a single blog entry! There are 2 accessible blog entries with mass harassment, and I think there must be more similar blog entries that were removed (because I was pinged for 4 times). Also, their other blog entries didn't contain any meaningful information. These blog entries only contained paragraphs in invalid languages or pictures with unknown meaning.
I believe that mass harassment and spam blog entr...
I'd like to ping [user:MikeMirzayanov,2024-06-28] to check this.
152.
ICPC Regionals — The Chennai flood and the Asia Director Background
----------
One of the regionals in India was scheduled to be held at [Hindustan University](http://icpc.hindustanuniv.ac.in/) at Chennai on December 17 — 18, 2015. Unfortunately Chennai was stuck by a [natural disaster](https://en.wikipedia.org/wiki/2015_South_Indian_floods). The entire south India received heavy rainfall and hence there was a lot of flooding at many places including Chennai around that time. Atleast 347 people died in the state of Tamil Nadu (where Chennai is located). The damage due to the floods overall in south India is estimated to be atleast USD 3 Billion. It is the costliest flood in 2015.
Ofcourse, due to this the Chennai regionals was postponed. I think everyone here will agree that this was an unexpected, unprecedent situation and hence it was entirely understandable that the regionals was postponed.
The Blog post
-------------
Dr. C J Hwang is the ICPC Asia director. In response to the above postponement he posted a blog article...
disastrous situation at Chennai. It is really sad that decisions are taken like
this at such a prestigious, Comment: Wet pants and shoes, seriously? This seems like a minor water logging
not a major
153.
Happy Birthday, Codeforces! Dear Codeforces, <img src="/predownloaded/32/a6/32a6b841dd1f306d9f2f1b232ef710ed9a27c350.jpeg" style="float:right; margin:0 1em 1em 1em;"/>
Happy 10th anniversary from Yandex! Over the past ten years, you have become more than just a platform for competitions having developed into a community hub where people can socialize, discuss interesting tasks and make useful contacts — a large part of life for every competitive programmer. Keep it up – continue growing and developing!
Codeforces, with its immense contribution to competitive programming, is very important to the entire community of sport programmers around the world. Yandex actively supports programming contests and competitions and appreciates what Codeforces does, having benefitted from a large number of talented employees with a background in competitive programming. It wouldn’t be the same without this community.
We would also like to use this opportunity to thank [user:MikeMirzayanov,2020-04-01] for Polygon, which ...
We would also like to use this opportunity to thank
[user:MikeMirzayanov,2020-04-01] for Polygon
154.
A simple introduction to "Segment tree beats" Hi, I’d like to introduce a simple trick about segment tree in this blog as I promised in [this comment](http://codeforces.me/blog/entry/54750?#comment-387957). Sorry for the long delay, as a sophomore in Peking University, I've just finished a tired semester and a painful final exam. And now I finally have enough time to do a simple introduction to this interesting algorithm.
It may be a huge project for me since my English is not good. I think I will finish this blog in several steps and I will try to finish it as soon as possible :)
In China, all of the 15 candidates for the Chinese National Team are asked to write a simple research report about algorithms in informatics Olympiad, and the score will be counted in the final selection. There are many interesting ideas and algorithms in these reports. And I find that some of them are quite new for competitors in CF although they are well known in China from the final standings of some recent contests. For example, In the last co...
I think most of the competitors’ templates of the lazy tag is like this ($[l,
r]$ is the node's, like this:
155.
Psyho Challenge: Where are your likes? Hello [user:Psyho,2022-12-21], this is Mike. We are happy to help with likes. So, Codeforces?
He waits for your likes here: https://twitter.com/FakePsyho/status/1605570944537280512?t=MV0OksdeUb8FXKApncDZcw&s=19
I believe that 40 facts are too few for us. We need more!
<img style="width:400px;" src="/predownloaded/e6/c8/e6c8915ea59e3b7b87bcf37af704221af1b66c5d.png"/>
156.
April Fools Day Contest 2024 Hello, <s>April Fools lovers</s> Codeforces!
We, [user:AlperenT,2024-03-26], [user:flamestorm,2024-03-26], [user:toxicpie9,2024-03-26], and [user:willy108,2024-03-26], would like to invite everyone on Codeforces to [contest:1952]! The 12th April Fools Day Contest will take place on [contest_time:1952]. This is a joke competition in which solving the problem is often easier than figuring out what the actual task is.
In this round, you will be given $n$ cool bugaboos where $-1 \le n \le \textbf{[REDACTED]}$ and **2** hours to solve them. The contest will use extended ICPC rules (no hacks, the standings are decided by the number of solved problems and penalty time earned on them). You can submit solutions in any language allowed by Codeforces, unless the problem says otherwise.
Please note that the round is **unrated** and that the penalty for a **wrong submission** is **10** minutes.
To get an idea of what the contest will look like, you can check out the contests of the pas...
funny responses like we did last year.
Announcement of April Fools Day Contest 2024
157.
[Repost] "Justice may be delayed, but it cannot be absent": New Evidence on
NXIST's Cheating Scandal
[Repost] "Justice may be delayed, but it cannot be absent": New Evidence on NXIST's Cheating Scandal _Original post: [「正义可以迟来但不能缺席」：关于 NXIST 的一些新证据](https://zhuanlan.zhihu.com/p/620297880)_
_Translated by GPT-4 with some adjustment. Please inform me if there are any mistakes._
_All links to the invalid repository have been redirected to the backup repository._
**This article provides a logically complete set of evidence, which does not involve any non-public internet resources, regarding the "suspected cheating" incident involving the ICPC Yinchuan Station and ICPC Shenyang Station in 2021. By discovering the suspected GitHub account ([NaokiLH](https://github.com/NaokiLH), renamed to https://github.com/brokenTarget) of a team member from Ningxia Institute of Science and Technology (NXIST) TS 1 team, Lan Hao, two years ago, and by mining and analyzing the commit records of his algorithm competition repo, we have obtained direct evidence that at least 4 questions from the 2021 Yinchuan regional contest set and at least 6 questions (including scrapped questions) from the 2021 Sh...
contest, and some problems like Jailbreak have not appeared to this day.
158.
You won't believe how this simple trick defeated the unexplained bug destroying every top LGM ### TL;DR
Currently people are discussing a slowdown bug on Codeforces that seems to happen randomly, and can cause code to run 100x slower and get TLE. More details in [user:pajenegod,2024-03-03]'s [blog post](https://codeforces.me/blog/entry/126654).
In this article, I present a mitigation: add the following to your code.
~~~~
#include <windows.h>
void *operator new(size_t size) {
if (void *ptr = HeapAlloc(GetProcessHeap(), 0, size ? size : 1)) return ptr;
throw std::bad_alloc{};
}
void operator delete(void *ptr) {
HeapFree(GetProcessHeap(), 0, ptr);
}
~~~~
<spoiler summary="If you use malloc/free in C++ (you shouldn't), also change them.">
~~~~
void *my_malloc(size_t size) {
if (void *ptr = HeapAlloc(GetProcessHeap(), 0, size ? size : 1)) return ptr;
throw std::bad_alloc{};
}
void my_free(void *ptr) {
HeapFree(GetProcessHeap(), 0, ptr);
}
~~~~
</spoiler>
<spoiler summary="If you use aligned new/delete in C++, also change the...
Subscribe for more posts like this where i solve CP bugaboos with the most
absurd techniques
159.
New contest format?! Stream feat. tourist, Um_nik Hey all!
We've created a new 1v1 programming contest format: **Lockout**. Like in most contests, each round has a set of problems and contestants work to solve them as quickly as they can. In Lockout, however, contestants compete head-to-head and only the first contestant to solve each problem gets the points. Contestants can work on problems in any order, so speed and strategy are crucial to avoid getting sniped! The head-to-head action also makes the contest much more exciting for viewers.
We ran the first edition of Lockout at TCO Finals last month as a double-elimination bracket [tournament](https://challonge.com/lockout0). All of the finalists who were available competed (and even some of the problem writers) and we got to see a lot of exciting back-and-forth matches! As you can see though, there's still one set left to play. So we'll be streaming **Grand Finals of Lockout 0 featuring [user:tourist,2019-12-28] vs. [user:Um_nik,2019-12-28]** at [9:30 AM PST](https://www.timea...
We've created a new 1v1 programming contest format: **Lockout**. Like in most
contests, each round
160.
Codeforces Round #723 (Rated for Div 2) Hello again Codeforces!
[user:Errorgorn,2021-05-25], [user:Oolimry,2021-05-25] and [user:ILoveIOI,2021-05-25] are glad to invite you to participate in [contest:1526] which will be held at [contest_time:1526]. The round will be **rated for the participants with a rating lower than 2100**. Participants from the first division are also welcomed to take part in the competition but it will be unrated for them.
You will be given **2 hours and 30 minutes** to solve **6 questions**. One of the puzzles is interactive, please read [the guide of interactive](https://codeforces.me/blog/entry/45307) [problems](https://en.wikipedia.org/wiki/Binary_search_algorithm) before the contest.
We would like to thank:
- Our lord and saviour [user:AntontrygubO_o,2021-05-25] for amazing coordination and rejecting our poopoo tasks
- [user:dantoh000,2021-05-25] and [user:tqbfjotld,2021-05-25] for roasting our enigmas and testing them before they were prepared
- our many testers [user:balbit,2021-05...
We would like to thank:
Announcement of Codeforces Round 723 (Div. 2)
161.
My own algorithm — offline incremental strongly connected components in O(m*log(m)) Hello Codeforces!
As all of you know, there are so many known algorithms named after people who invented them — from the easiest ones, like Dijkstra, to the harder ones, like Berlekamp–Massey algorithm. These algorithms were innovative when they were invented, so of course, it's good that they are named after their inventors.
But, today, I've seen a blog about the solution to the problem "compute LCS of two strings in time $O((n + k) \cdot \log(k))$, where $n$ is the sum of lengths of the strings, and $k$ is the number of pairs of matching positions in them". To be honest, it a bit pissed me off that even this algorithm is named after its creators. Is it ok to name the solution to every possible problem after its author? I won't judge it. Anyway, I want my very own Radecki algorithm, so let me give it a try. If anyone has ever heard about it — it's cool. Let me know, and it'll be just another helpful blog on Codeforces.
Let's imagine the following problem: there is...
; from the easiest ones, like Dijkstra, to the harder ones, like
Berlekamp–Massey algorithm. These
162.
Blogewoosh #7 Hello, codeforces!
Long time no see, right? So maybe it's a good idea to try to return to my blogs slowly. This time the blog will be about a trick, which usually isn't necessary to solve a task, but can be useful to make implementation much more comfortable.
Let's look at [this](https://codeforces.me/contest/932/problem/F) problem. It is about some DP on a tree in which we have to use convex hull trick to improve the complexity. The task requires merging two convex hulls with "smaller to bigger" trick. I recommend you to read the statement before reading the rest of the blog (and the [editorial](https://codeforces.me/blog/entry/57796) if you don't know how to solve it).
[cut]
$ $
So, how to merge two convex hulls? If we keep them in something like the set from C++, then we can just take every element from the smaller one and try to add it to the bigger one. Unfortunately, adding one element in this way requires some ifology and care. It's hard to implement it, especiall...
So, how to merge two convex hulls? If we keep them in something like the set
from C++, then we can
163.
"Algorithms in Depth" Stream Series: Kicking Off With FFT Hi Codeforces!
I’ve long believed that it's _really_ worth revisiting the same competitive programming topics multiple times.
The first time, you’re just trying to survive and maybe grasp the basics. The second time, you’ve solved some problems, you know where you got confused, and you're ready to absorb more.
By the third time, you’ve seen and solved enough to start appreciating the subtleties — the tricks, advanced applications, deeper meaning.
But most lectures I’ve seen are designed as a one-size-fits-all — trying to cover everything from scratch _and_ dive into the advanced stuff in one go. That’s rarely optimal.
Take binary search. I once talked about it for six hours straight. A beginner at that lecture would’ve probably quit CP immediately. And that’s ok. Beginners don’t need all that. But the truth is, there _is_ that much depth to explore even in seemingly simple topics like binary search — once you're ready.
Yet many people only see the beginner version once and mo...
There are many more topics like that: segment trees, square root decomposition,
MSTs, etc.
164.
CodeTON Round 4 (Div. 1 + Div. 2, Rated, Prizes!) Hello, Codeforces!
I'm very glad to invite you to participate in [CodeTON Round 4 (Div. 1 + Div. 2, Rated, Prizes!)](https://codeforces.me/contests/1810), which will start on [contest_time:1810].You will be given **8 problems** and **2 hours** to solve them. The round will be rated for everyone.
I'd like to give my sincere thanks to:
* [user:74TrAkToR,2023-03-29] for the wonderful coordination!
* [user:Sulfox,2023-03-29] for the help with the statements.
* [user:Suika_predator,2023-03-29], [user:SSerxhs,2023-03-29], [user:LLL_2820,2023-03-29], [user:xay5421,2023-03-29], [user:NemanjaSo2005,2023-03-29], [user:applese,2023-03-29], [user:bxhnb,2023-03-29], [user:fr200110217102,2023-03-29], [user:Sulfox,2023-03-29], [user:CCSU_DZX,2023-03-29] [user:PaperCloud,2023-03-29], [user:CedeaT0,2023-03-29], [user:Kath1031,2023-03-29], [user:jcccc,2023-03-29], [user:dyxg,2023-03-29], [user:tzc_wk,2023-03-29], [user:xiaoziya,2023-03-29], [user:qiqi20021026,2023-03-29], [user:Pointy,20...
I'd like to give my sincere thanks to:
Announcement of CodeTON Round 4 (Div. 1 + Div. 2, Rated, Prizes!)
165.
Codeforces Round 872 (Div. 1 & 2) Hello Codeforces! （你好，代码部队）
We are glad to invite you to participate in [contest:1824] and [contest:1825] on [contest_time:1824]. **Please note the unusual start time.**
This round is rated for **all the participants**. You will be given 5 problems (one of which is divided into two subtasks) and 2 hours to solve them in each division.
Assume you are [user:YuezhengLing,2023-04-30], and in this round, your best friend [user:luotianyi,2023-04-23] will give you several problems to solve.
([user:luotianyi,2023-04-23] is the left one and [user:yuezhengling,2023-04-30] is the right one)
We would like to thank:
1. [user:RedLycoris,2023-04-22], [user:CrTsIr,2023-04-22], [user:Gyy_cj,2023-04-22], [user:SanweiTreap,2023-04-22] and [user:Gezhiyuan,2023-02-06] for writing the problems.
2. [user:Artyom123,2023-04-22] for coordinating the round.
3. [user:MikeMirzayanov,2023-05-06] for great Codeforces and Pol...
We would like to thank:
Announcement of Codeforces Round 872 (Div. 1)
Announcement of Codeforces Round 872 (Div. 2)
166.
Codeforces Round #754 (Div. 2) Henlo Codeforces!
We are very glad to invite you to the [contest:1605], which will be held on [contest_time:1605]. This round will be rated for participants with a rating less than $2100$. For higher rated participants, we challenge you to solve problem F ;)
All the problems were authored and prepared by [user:Ashishgup,2021-11-08], [user:the_hyp0cr1t3,2021-11-08], [user:ExplodingFreeze,2021-11-08] and me. We have tried our best to create an interesting problemset with clear statements. Hope you enjoy the round :D
You will be given $6$ problems and $2$ hours to solve them.
We would like to thank:
- [user:antontrygubo_O,2021-11-08] for his amazing coordination and help in improving our problems.
- Our army of testers: [user:Um_nik,2021-11-08], [user:kefaa2,2021-11-08], [user:1-gon,2021-11-08], [user:_runtimeTerror_,2021-11-08], [user:Jeel_Vaishnav,2021-11-08], [user:AmShZ,2021-11-08], [user:errorgorn,2021-11-08],
[user:ijxjdjd,2021-11-08], [user:Utkarsh.25dec,2021-11...
We would like to thank:
Announcement of Codeforces Round 754 (Div. 2)
167.
Changes in CodeChef Problemsetting Hello Codeforces!
For the last half a year I have been working on CodeChef and was proposing ideas on how to change the problemsetting practices (hopefully, for the best). For some time the changes were in brainstorming phase, but as of now many of them are implemented. Today we are ready to share the information with you. But first I would like to approach people who are going to close this blog without reading:
If you are not interested in CodeChef contests at all, I think you should change your mind. Monthly short contests (Cook-Off and Lunchtime) have interesting quality problems, on par with Codeforces in my opinion (not AtCoder level yet, but who are?). Long and Starters are more targeted to newbie participants, with more classical educational problems.
If you are interested in setting problems for regular contests, you might want to do it on CodeChef. Some reasons:
- You don't have to set the whole round, as we work on per problem basis.
- This may be especially...
What changes would you like to see on CodeChef, either in problemsetting or in
general? Please help
168.
Codeforces Round #738 (Div. 2) Hi Codeforces!
We are glad to invite you to our first Codeforces Round [contest:1559] which will be held on [contest_time:1559]. This round will be **rated for participants with rating less than 2100**. We will be glad to see participants from the first division to join out of competition as well!
In this round, as the best friend of Mocha's, you are going to help her to solve the problems she meets.
The problems are prepared by [user:2sozx,2021-08-12], [user:JJLeo,2021-08-12], [user:Serval,2021-08-12], [user:CalvinJin,2021-08-12] and me. We hope you will enjoy the round. :P
We would like to thank:
- [user:1-gon,2021-08-12] for awesome coordination of this round and helping us improve our problems.
- [user:Toxel,2021-08-12] for excellent testing and discussion of this round.
- [user:cppascalinux,2021-08-12], [user:PurpleCrayon,2021-08-12], [user:ijxjdjd,2021-08-12], [user:m371,2021-08-12], [user:qxforever,2021-08-12], [user:fixed_lxy,2021-08-12], [user:HKvv,2021...
We would like to thank:
Announcement of Codeforces Round 738 (Div. 2)
169.
Codeforces Round #270 Do you want to win a T-shirt? Do you want to learn how to design tasks for programming contest? Do you want to solve 7 tasks in 2.5 hours?
So Codeforces Round #270 is right for you. It was designed by me in California, Assembled in polygon (so Thank you [user:MikeMirzayanov,2014-09-26] for the system and [user:Gerald,2014-09-26] for organize and testing), will start on [regular time this Sunday](http://www.timeanddate.com/worldclock/fixedtime.html?day=28&month=9&year=2014&hour=19&min=30&sec=0&p1=166), don't miss it!
The organizers of **<a href="/blog/entry/13929">Marathon24</a>** decided to present gifts to the best finishers of the round! Best 25 participants will get Marathon24 tshirts! Thanks!
<center>
<img src="http://www.elmostshirts.com/images/tshirts.jpg"/>
<br><small>It is just an image to attract your attention. Real tshirts will be designed specially for Marathon24!</small>
</center>
There are some articles introduced how to become a problem setter, like [Pro...
**Update2** : Contest ended. Thanks for participate! Any comment is welcome (you
like the task
Announcement of Codeforces Round 270
170.
Codeforces Round 975 (Div. 1, Div. 2) Ciao, Codeforces! We're glad to invite you to take part in [contest:2018] and [contest:2019], which will start on [contest_time:2018]. You will be given **6 problems** and **2 hours and 30 minutes** to solve them in both divisions. Some problems will be divided into subtasks.
**UPD: the time has been changed to [contest_time:2018], which is different from the time announced before. Please note the unusual starting time.**
This round is based on [Italian Olympiad in Informatics (OII) 2024](https://codeforces.me/blog/entry/133905).
The problems were authored by [user:lorenzoferrari,2024-09-22], [user:wksni,2024-09-22] and me.
We would like to thank
- [user:KAN,2024-09-22] for his nutella coordination;
- [user:Alexdat2000,2024-09-27] for Russian translation;
- [user:bortoz,2024-09-22], [user:collodel,2024-09-22], [user:dp_1,2024-09-22], [user:franfill,2024-09-22], [user:franv,2024-09-22], [user:harniver,2024-09-22], [user:jamesbamber,2024-09-22], [user:Kaey,2024-09-22], ...
We hope you'll like the problemset!, We would like to thank
Announcement of Codeforces Round 975 (Div. 1)
Announcement of Codeforces Round 975 (Div. 2)
171.
Blogewoosh #4 Hello, codeforces!
It's time to continue the series of Polish tasks. I've decided to write about my own task one more time. Its name is "cook" ([you can submit here](https://szkopul.edu.pl/problemset/problem/9NFtPM59qGWa7wdn570ifuP0/site/?key=statement)). The task isn't very hard, but it uses cute (in my opinion) trick. The statement goes as follows:
There is a cook in a restaurant. He has $n$ ($1 \leq n \leq 10^6$) orders which he must fill. Every order is a piece of paper, and all orders are speared on a spindle (sharp stick with pierced pieces of paper) in a fixed order which cannot be changed. Normal cook would just take orders one by one from the top of the spindle and fill them in this order, but the cook in this task has supernatural cooking powers and can combine orders to fill them faster. In particular, if at some moment there are $k$ out of $n$ orders still on the spindle, he can choose one of three options:
$---$ He can take the topmost piece of paper and fill this...
one. Isn't it too easy? Did you like the trick with decreasing the memory? Let
me know in the comments
172.
[Tutorial] The DFS tree and its applications: how I found out I really didn't understand bridges #### Introduction
This is a tutorial/exploration of problems that can be solved using the "DFS tree" of a graph.
For a way too long time, I didn't really understand how and why the classical algorithm for finding bridges works.
It felt like many tutorials didn't really explain how it works, kind of just mentioned it in passing and quickly
just moved on to implementation. The day someone explained what the DFS tree is, I finally understood it properly.
Before, it took me ages to implement bridge-finding properly, and I always had to look up some detail. Now I can
implement it at typing speed.
But more importantly, I began to see how the same techniques can be used to solve graph problems that have more or
less nothing to do with bridges. The thing is, when you have a black box, you can only ever use it as a black box.
But if you have a box that you understand well, you can take it into pieces, repurpose the pieces for completely
different things, all without getting lost...
Here is an animation of what calling `visit(1)` looks like., Step 2 can be done like this, for example:
173.
Codeforces Round #746 (Div. 2) صباحو (Good Morning), Codeforces!
I'm glad to invite you to [contest:1592], which will be held on [contest_time:1592].
This round is rated for the participants with rating lower than 2100.
You will be given **6 problems** and **2 hours** to solve them. All problems were prepared by me and [user:Bakry_,2021-10-01].
One of the problems will be interactive. So, it is recommended to read the [guide on interactive problems](https://codeforces.me/blog/entry/45307) before the round.
I would like to thank:
- [user:antontrygubO_o,2021-10-01], for a lot of things (awesome coordination, suggestion for one of the tasks, helping in preparation, rejecting only not interesting tasks, a lot of useful discussions).
- [user:KostasKostil,2021-10-01] for one of the tasks and [user:antontrygubO_o,2021-10-01] for preparing it.
- [user:YahiaSherif,2021-10-01], [user:ZeyadKhattab,2021-10-01], [user:mahmoudbadawy,2021-10-01], [user:Uzumaki_Narutoo,2021-10-01], [user:_zaher_,2021-10-01]...
I would like to thank:
Announcement of Codeforces Round 746 (Div. 2)
174.
Regarding AtCoder’s Ban Policy and the Recent ARC222 Ban In ARC222, held last week, we banned a user who had been in first place at one point during the contest.
This has led to discussion about AtCoder’s ban system, so I would like to explain several points.
## General Policy
The people responsible for making ban decisions are well aware that there may be strong competitors who have not participated in many contests before. We do not ban users merely because they achieve good results.
That said, we cannot say that the possibility of a mistaken ban is completely zero. Recently, every time we hold a contest, tens to hundreds of bad actors have attempted to achieve high rankings using AI. Their methods have also become increasingly sophisticated, and we spend a great deal of time and effort making these judgments. As a result, we cannot completely deny the possibility that legitimate participants may be banned by mistake.
However, for users who have already participated in dozens of contests, we conduct thorough investigations a...
This has led to discussion about AtCoder’s ban system, so I would like to
explain several points.
Codeforces (c) Copyright 2010-2026 Mike Mirzayanov
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