numbers from $1$ to $3$ so that people can understand what are the best topics
for them. The, - $1$ — If your rating is $1600 - 1899$ - $2$ — If your rating is $1900 - 2399$
- $3, Lastly, I tried to state the difficulty of each topic by numbers from $1$ to $3
$ so that people can
→ Pay attention
→ Top rated
| # | User | Rating |
|---|---|---|
| 1 | Benq | 3676 |
| 2 | Kevin114514 | 3659 |
| 3 | jiangly | 3640 |
| 4 | turmax | 3450 |
| 5 | tourist | 3408 |
| 6 | strapple | 3360 |
| 7 | Um_nik | 3354 |
| 8 | squareOf105 | 3348 |
| 9 | maroonrk | 3312 |
| 10 | idontwannadothis…e | 3191 |
→ Top contributors
| # | User | Contrib. |
|---|---|---|
| 1 | Qingyu | 154 |
| 2 | nik_exists | 151 |
| 3 | maspy | 149 |
| 4 | AmShZ | 146 |
| 5 | Um_nik | 143 |
| 6 | Errichto | 139 |
| 7 | BledDest | 136 |
| 8 | adamant | 135 |
| 9 | maroonrk | 133 |
| 10 | cry | 130 |
→ Find user
→ Recent actions
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.
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...
2.
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...
3.
Codeforces Global Round 5 <img src="/predownloaded/2e/e2/2ee2f0f95f05aae6ca6f47812dae2dc2aabf277b.png" style="width:200px; float:right; margin: 0 1em 1em 1em"/>
Hey!
On [contest_time:1237] we will host [contest:1237].
It is the fifth 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.
The round will last for 2 hours 30 minutes, 8 problems are waiting for you, and one of them will be proposed in two versions.
Scoring distribution: **500 — 750 — (750 + 750) — 2000 — 2500 — 3000 — 3750 — 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 fi...
. [user:Radewoosh,2019-10-16] 2. [user:Petr,2019-10-16] 3.
[user:300iq,2019-10-16] 4. [user:ecnerwala,2019-10-16, 1. [user:Radewoosh,2019-10-16] 2. [user:Petr,2019-10-16] 3.
[user:300iq,2019-10-16] 4, ] 3. [user:300iq,2019-10-16] 4. [user:ecnerwala,2019-10-16] 5.
[user:RomaWhite,2019-10-16]
Announcement of Codeforces Global Round 5
4.
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...
Div1F - 2 * 3 = 6 USD, haven't received my previous contest's payment yet which happened 3 months ago!, . [user:djq_cpp,2021-10-30] 3. [user:Petr,2021-10-30] 4.
[user:DmitryGrigorev,2021-10-30] 5, 1. [user:Rolling_Code,2021-10-30] 2. [user:Pecans,2021-10-30] 3.
[user:fangzhijina2020,2021-10-30, 1. [user:tourist,2021-10-30] 2. [user:djq_cpp,2021-10-30] 3.
[user:Petr,2021-10-30] 4
Announcement of Codeforces Round 752 (Div. 1)
Announcement of Codeforces Round 752 (Div. 2)
5.
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 ...
If $n$ is odd, there must be some $\lfloor \frac{a_i}{m} \rfloor \geq 3$. In
this case, you can, ][mask1][mask2]%=MOD; for (int m=0;m<8;m++) { bool flag=false; for (int j=0;j<3
;j, }{m} \rfloor \geq 3$. In this case, you can draw a beautiful picture using such
algorithm: * Sort
Tutorial of Codeforces Round 810 (Div. 1)
Tutorial of Codeforces Round 810 (Div. 2)
6.
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...
7.
VK Cup 2021 — Отбор (Engine) и Codeforces Round #733 (Div. 1 + Div. 2) Привет, Codeforces!
[contest:1544] уже совсем скоро, не пропустите начало: [contest_time:1544]. Это соревнование предназначено для тех, кто решил хотя бы 7 задач из 8 в квалификационном раунде [VK Cup 2021](https://codeforces.me/vkcup2021). Раунд будет рейтинговым для всех.
Но даже если вы не регистрировались на VK Cup 2021, добро пожаловать на объединенный Div. 1 + Div. 2 раунд [contest:1530], который начнётся в то же время. Он также будет рейтинговым и открытым для обоих дивизионов.
Все задачи были придуманы и подготовлены мной. Большое спасибо всем, без кого этот раунд не смог бы состояться: [user:PavelKunyavskiy,2021-07-16], [user:KAN,2021-07-16], [user:lperovskaya,2021-07-16], [user:ksun48,2021-07-16], [user:Sert,2021-07-16], [user:Aleks5d,2021-07-16], [user:MikeMirzayanov,2021-07-16].
Участникам будет предложено 8 задач и 3 часа на их решение. Рекомендуем прочитать условия всех задач. Удачи!
Среди участников закрытого отборочного раунда, 64 лучших участника получа...
problems and 3 hours to solve them. It is recommended to read all the
problems. Good luck! **UPD, ,2021-07-16]. Участникам будет предложено 8 задач и 3 часа на их решение.
Рекомендуем прочитать, - 1-е место — 300 000 рублей; - 2-е — 200 000; - 3-е — 100 000; - 4-е — 50 000;
- и 5-е, 1. [user:Um_nik,2021-07-17] 2. [user:Petr,2021-07-17] 3.
[user:Endagorion,2021-07-17] 4, 1. [user:jiangly,2021-07-17] 2. [user:ecnerwala,2021-07-17] 3.
[user:Radewoosh,2021-07-17] 4, You will be given 8 problems and 3 hours to solve them. It is recommended to
read all the problems, Участникам будет предложено 8 задач и 3 часа на их решение. Рекомендуем
прочитать условия всех
Announcement of VK Cup 2021 - Elimination (Engine)
8.
Hello 2018 Hello 2018!
If you're still thinking what to do on the eighth day of year 2018, pay attention! The first round for both divisions of the new year starts on January 8 at 17:35 Moscow time ([what about other timezones?](https://www.timeanddate.com/worldclock/fixedtime.html?msg=Hello+2018&iso=20180108T1735&p1=3396&ah=2&am=30)).
Four important components of Hello 2018 will be the same as in Good Bye 2017:
- Div1 + Div2 combined
- 8 problems
- 2 hours 30 minutes
- Rated
But there will also be a substantial difference:
- Different problems
The problems of this round have been proposed and prepared by [user:YakutovDmitriy,2018-01-06], [user:BudAlNik,2018-01-06] and myself.
Thanks to everyone without whom this round wouldn't be possible as well: [user:AlexFetisov,2018-01-06], [user:Golovanov399,2018-01-08], [user:KAN,2018-01-06], [user:MikeMirzayanov,2018-01-06], [user:PavelKunyavskiy,2018-01-06], [user:qwerty787788,2018-01-06], [user:VArtem,2018-01-07], [user:winger,20...
. [user:desert97,2018-01-08] 3. [user:yosupo,2018-01-08] 4.
[user:dotorya,2018-01-08] 5, 1. [user:Um_nik,2018-01-08] 2. [user:desert97,2018-01-08] 3.
[user:yosupo,2018-01-08] 4
Announcement of Hello 2018
9.
VK Cup 2017 Round 3 + Codeforces Round #412 (Div. 1 + Div. 2) Hi everyone!
The last elimination round of VK Cup 2017, Round 3, will take place on May 7 at 18:35 MSK ([check your timezone here](https://www.timeanddate.com/worldclock/fixedtime.html?msg=VK+Cup+Round+3&iso=20170507T1835&p1=352&ah=3)), along with separate Codeforces Round #412 for both divisions. All three rounds will be three hours long, and all three rounds will be rated.
The contest "VK Cup 2017 — Round 3" is for teams qualified from Round 2 or Wildcard Round 2. The top 20 teams will advance to the final which will be held in July 2017 in Saint Petersburg!
Huge thanks to [user:KAN,2017-05-06], [user:qwerty787788,2017-05-06], [user:PavelKunyavskiy,2017-05-06], [user:AlexFetisov,2017-05-07], [user:MikeMirzayanov,2017-05-06], and VK company for making this round possible.
Codeforces will be the main character of most problems. Don't forget that it's useful to read the statements of all the problems.
Good luck!
As we're in year 2017, the scoring will obviously be stat...
VK Cup 2017 Round 3 + Codeforces Round #412 (Div. 1 + Div. 2), **UPD 3.** Congratulations to the winners!, **UPD 3.** Поздравляем победителей!, +Round+3&iso=20170507T1835&p1=352&ah=3)). Параллельно с ним на тех же задачах
состоится Codeforces Round, ,2017-05-07] 3. [user:RomaWhite,2017-05-07], [user:witua,2017-05-07] 4.
[user:YakutovDmitry,2017-05, /fixedtime.html?msg=VK+Cup+Round+3&iso=20170507T1835&p1=352&ah=3)). Параллельно
с ним на тех же задачах состоится, 1. [user:Petr,2017-05-07] 2. [user:yosupo,2017-05-07] 3.
[user:rng_58,2017-05-07] 4. [user:uwi, 1. [user:ltaravilse,2017-05-07] 2. [user:btk15049,2017-05-07] 3.
[user:RCG,2017-05-07] 4, Hi everyone! The last elimination round of VK Cup 2017, Round 3, will take
place on May 7 at 18, The contest "VK Cup 2017 — Round 3" is for teams qualified from Round 2 or
Wildcard Round 2. The, The last elimination round of VK Cup 2017, Round 3, will take place on May 7 at
18:35 MSK ([check, VK Cup Round 3:, Соревнование "VK Cup 2017 — Раунд 3" предназначено для команд,
квалифицировавшихся из Раунда 2 или
10.
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...
1. [user:tourist,2020-07-04] 2. [user:scott_wu,2020-07-04] 3.
[user:yosupo,2020-07-04] 4
Announcement of Codeforces Global Round 9
11.
Codeforces Round 1119 (Div. 3) <pre><code class="cpp">#include "codeforces.h"
#include <chrono>
#include <vector>
using namespace std;
using namespace std::chrono_literals;
using namespace cf;
int main() {
Blog announcement("Codeforces Round 1119 (Div. 3)");
announcement << "Hello Codeforces!\n";
announcement << "I am delighted to invite you to participate in " << </code>[contest:2259]<code class="cpp"> << "!\n";
announcement << "The problems were written and prepared by " << </code>[user:nik_exists,2026-09-01]<code class="cpp"> << ".\n\n";
announcement << ContestInfo {
.start_time = </code>[contest_time:2259]<code class="cpp">,
.duration = 2h + 15min,
.number_of_problems = 7,
.subtasks = true // possibly not adjacent
};
announcement << RatingRules {
.rated_for_rating_below = 1600,
.unrated_registration_allowed = true,
.trusted_max_rating_...
Codeforces Round 1119 (Div. 3)
Announcement of Codeforces Round 1119 (Div. 3)
12.
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...
. [user:ecnerwala,2024-01-07] 2. [user:ksun48,2024-01-07] 3.
[user:VivaciousAubergine,2024-01-07] 4, 1. [user:ecnerwala,2024-01-07] 2. [user:ksun48,2024-01-07] 3.
[user:VivaciousAubergine,2024-01-07
Announcement of Hello 2024
13.
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
14.
Codeforces Round #639: the round is rescheduled Hello, Codeforces!
Unfortunately, I report that I decided to reschedule this round. I'm afraid that something might go wrong. Some strange database behavior has appeared, which leads to slow operation in completely unexpected places (it never was and should not be). I will work hard to fix it. My recent tests show that this can dramatically increase judging time (and leads to a huge queue). I don't want to risk the efforts of the writer, the coordinator, testers, and your time. Sorry about it, I was trying to fix it for many hours but I need more time. Hope it will be a great round!
Mike.
**UPD 1:** I temporarily rescheduled it on May, 6. But I'll discuss the date with the writer and coordinator and reschedule it again to the date they want.
**UPD 2:** [user:Monogon,2020-05-04] approved that [contest_time:1345] is good new date and time for the round. See you in 3 days!
**UPD 3:** It seems I've fixed the issue. It turned that it was connected with [week ago incident](htt...
for the round. See you in 3 days!, **UPD 3:** It seems I've fixed the issue. It turned that it was connected with
[week ago incident, ] approved that [contest_time:1345] is good new date and time for the round.
See you in3 days
15.
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...
Div. 1 and 31 Div. 2-only contests, to be precise! Additionally, 18 Div. 3, 4
Div. 4, and 13, , [user:Vladosiya,2025-12-31] managed all Div. 3 and Div. 4 rounds, with
[user:cry,2025-12-31] and, . 2-only contests, to be precise! Additionally, 18 Div. 3, 4 Div. 4, and 13
Educational rounds made
16.
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]...
.2: 1. [user:turmax,2021-07-29] 2. [user:karry5307aknoi2021,2021-07-29] 3.
[user:SSRS_,2021-07, 1. [user:karry5307aknoi2021,2021-07-29] 2. [user:X_qaeq,2021-07-29] 3.
[user:MiFaFaMYY,2021-07-29, 1. [user:turmax,2021-07-29] 2. [user:karry5307aknoi2021,2021-07-29] 3.
[user:SSRS_,2021-07-29] 4
Announcement of Codeforces Round 735 (Div. 2)
17.
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;"/>...
best of the best to organize rounds! **News #3.** And that's not all! We're
continuing to develop, **News #3.** And that's not all! We're continuing to develop unique
competitions in collaboration
18.
Hello 2019 Hello coders! I hope that you are enjoying the New Year as much as me. To make its beginning even greater, Codeforces is going to host a contest and I will be an author of all tasks. Hello 2019 will take place on [Friday](https://www.timeanddate.com/worldclock/fixedtime.html?day=4&month=1&year=2019&hour=17&min=35&sec=0&p1=166).
Using the opportunity, I want to thank to:
- [user:lewin,2019-01-02] and [user:mnbvmar,2019-01-02] for testing the round.
- [user:mnbvmar,2019-01-02] for indescribably helpful discussions about problems.
- [user:_kun_,2019-01-02] and [user:KAN,2019-01-02] for round coordination and help with preparation.
- [user:MikeMirzayanov,2019-01-02] for such great platforms (you know which ones :P).
The round will consist of 8 problems and you will be given <s>two and a half</s> three hours to solve them. Yes, the round will be **rated**.
There will be no interactive problems, but if you want you can read [this](https://codeforces.me/blog/entry/45307) docum...
**UPD4:** The round will be **3 hours** long., 1. [user:V--o_o--V,2019-01-05] 2. [user:ecnerwala,2019-01-05] 3.
[user:tourist,2019-01-05] 4
Announcement of Hello 2019
19.
Codeforces Round #635 _Riichi...Tsumo! 2 han 2000 points!_
Hi! Have you ever heard of the game called _Mahjong Soul_? It is a Japanese Mahjong game that is famous for the adorable characters.
We are excited to invite you to take part in [Codeforces Round #635](https://codeforces.me/contests/1336,1337), where you can help the characters in trouble. This round will be held on [contest_time:1336]. Most importantly, it is **rated** for both divisions!
Each division will be given **6 problems** and you will have **2.5 hours** to solve them. An interactive problem may be found in this round. If you are not familiar with interactive problems, you can learn about them [here](https://codeforces.me/blog/entry/45307).
The problems were prepared by [user:EternalAlexander,2020-04-14], [user:ustze,2020-04-14] and me [user:Sooke,2020-04-14].
We sincerely thank [user:isaf27,2020-04-14] for reviewing and coordinating the round, and ...
solve 1F!) 3. [user:Golovanov399,2020-04-15] (First to solve 1B!) 4.
[user:Endagorion,2020-04-15, **UPD 3:** Congratulations to the winners!, ] 3. [user:01191020csl,2020-04-15] 4. [user:DreamLoIita,2020-04-15] 5.
[user:hfyzw,2020-04-15] 6
Announcement of Codeforces Round 635 (Div. 1)
Announcement of Codeforces Round 635 (Div. 2)
20.
0 Problems Remaining [日本語記事(For Japanese Speakers)](https://maspypy.com/codeforces-all-ac)
----
Previous post: https://codeforces.me/blog/entry/150057
Submission: [submission:384239819]
2026/07/26 03:13:42 JST (2026/07/25 18:13:42 UTC)
I have solved every problem in the Codeforces PROBLEMSET except for those for which I determined that obtaining an accepted verdict is impossible.
# Scope
I considered the problems listed in the [PROBLEMSET](https://codeforces.me/problemset).
I have solved very few problems from the [GYM](https://codeforces.me/gyms). The only collection I worked through systematically was the PROBLEMSET.
I determined that it is theoretically impossible to obtain an accepted verdict for the following 31 problems:
[problem:775A], ([92101](https://codeforces.me/problemset/problem/921/01) ... [92114](https://codeforces.me/problemset/problem/921/14)), [problem:927A], [prob...
PROBLEMSET came within reach. Looking back, I think I started by trying to
complete all Div.3 problems, Looking back, I think I started by trying to complete all Div. 3 problems.
21.
Codeforces: серия обновлений (рейтинг, раунды) <img src="/predownloaded/21/a3/21a33b497ee8a34a678befc3f79745eb69a9cf07.png" style="float:right; margin: 0 1em 1em 1em;"/>
Привет, Codeforces!
Настало время новостей. Надеюсь, вам понравится!
* Мы немного опускаем границу рейтинга для оранжевых участников. Таким образом, теперь оранжевый рейтинг (и звание _мастер_) будет начинаться от 2100 единиц рейтинга.
* Нижняя граница рейтинга для _легендарных гроссмейстеров_ увеличена до 3000 единиц рейтинга. Таким образом, прямо сейчас в сообществе 14 участников соответствуют этому требованию. Настоящие легенды!
* Для раундов серии «только для Div. 2» увеличена верхняя граница рейтинга для официального участия. Теперь все, чей рейтинг менее 2100 (то есть до _кандидатов в мастера_ включительно) смогут зарегистрироваться на такие раунды для участия в конкурсе, их рейтинг будет обновлён по результатам раунда. Мы не планируем повышать сложность Div. 2 раундов. Практика показывает, что задачи таких раундов и сейчас интересны и полезны ...
(Div. 3). В таких раундах в официальные результаты будут попадать участники с
рейтингами менее 1600, * As an experiment, we plan to host rounds of simplified complexity (Div. 3).
In such rounds, the, * В качестве эксперимента мы планируем проводить раунды упрощенной сложности
(Div.3). В таких
22.
Codeforces Round #658 Hello, Codeforces!
I'm very glad to invite you to [contest:1381] and [contest:1382]. This contest will take place on [contest_time:1381]. In both divisions, you will have **2 hours** to solve **5 problems** (and one subtask). The score distribution will be announced closer to the start of the round.
Huge thanks to:
- [user:isaf27,2020-07-19] for excellent coordination of this round
- [user:Andreii,2020-07-19], [user:antontrygubO_o,2020-07-19], [user:Ari,2020-07-19], [user:DeadlyCritic,2020-07-19], [user:gamegame,2020-07-19], [user:growup974,2020-07-19], [user:hugopm,2020-07-19], [user:Kuroni,2020-07-19], [user:NooB_MB,2020-07-19], [user:Priyank,2020-07-19], [user:SecondThread,2020-07-19], [user:--Someone--,2020-07-19], [user:talibmohd,2020-07-19], [user:WNG,2020-07-19] for testing the problems and providing useful feedback.
- [user:MikeMirzayanov,2020-07-19] for providing Codeforces and Polygon platforms.
I've worked hard to ensure the pretests are short and the statement...
. [user:badger_champion,2020-07-21] 2. [user:Mai_madarchod_hu,2020-07-21] 3.
[user:rulai,2020-07-21] 4, 3. [user:KAN,2020-07-21], 3. [user:rulai,2020-07-21]
Announcement of Codeforces Round 658 (Div. 1)
Announcement of Codeforces Round 658 (Div. 2)
23.
Things I don't know I'm just in a mood to shitpost. Don't take it too seriously.
Things that I have heard of, but don't know (imagine how many things I haven't even heard of):
- Li-Chao Segment Tree
- Segment Tree Beats
- RMQ in $O(n)$/$O(1)$
- Any self-balancing tree except treap
- Link-cut tree
- Wavelet tree
- Mergesort tree
- Binomial heap
- Fibonacci heap
- Leftist heap
- Dominator tree
- 3-connected components in $O(n)$
- $k$-th shortest path
- Matching in general graph
- Weighted matching in general graph
- Preflow-push
- MCMF in $O(poly(V, E))$
- Minimum arborescence (directed MST) in $O(E \log V)$
- Suffix tree
- Online convex hull in 2D
- Convex hull in 3D
- Halfplane intersection
- Voronoi diagram / Delaunay triangulation
- Operation on formal power series (exp, log, sqrt, ...) (I know the general idea of Newton method)
- How to actually use generating functions to solve problems
- Lagrange Inversion formula
- [...
- Dominator tree - 3-connected components in $O(n)$ - $k$-th shortest path -
Matching in general, - Fibonacci heap - Leftist heap - Dominator tree - 3-connected components in
$O(n)$ - $k$-th, If you know at least 3 of these things and you are not red — you are doing it
wrong. Stop
24.
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-...
[contest_time:2178]. You will be given 9 problems and 3 hours to solve them.
At least one, [contest_time:2178]. You will be given 9 problems and 3 hours to solve them. At, **Update 3:** [Editorial](https://codeforces.me/blog/entry/149548), 1. [user:jiangly,2025-12-25] 2. [user:Benq,2025-12-25] 3.
[user:Kevin114514,2025-12-25] 4
Announcement of Good Bye 2025
25.
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...
1. India 2. Egypt 3. Brazil 4. United States 5. Bangladesh 6. Peru 7. Jordan 8.
Syria 9, Edit 3: A2 Online Judge is shut down now, and a static version of all ladders
and categories is, ](https://codeforces.me/blog/entry/47442)** Edit 3: A2 Online Judge is shut
down now, and a static
26.
Blogewoosh #1 Hello, codeforces!
The community wants so the community gets it! :D Here it is, my very first blog about tasks and algorithms. At the beginning I've decided to post my entries on codeforces, maybe I'll switch to something different if it becomes uncomfortable.
To pour the first blood I decided to choose a task from one of the old ONTAK camps. Task's name is "different words" ([you can submit here](https://szkopul.edu.pl/problemset/problem/wTy-sxQCIKry0Ml-6RvM0L78/site/?key=statement)). 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...
come up with something with $k^3$ in complexity, then it'll probably be
multiplied by $n$. $O(k^5)$ is, out to be hard, even if you can come up with something with $k^3$ in
complexity, then it'll
27.
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:
...
Codeforces Round 923 (Div. 3), round for the Div.3. Lately, I've been coming up with problem ideas less
frequently, but I don't, As usual for the Div.3 rounds:, On [contest_time:1927] will start [contest:1927], the next Codeforces round for
the Div.3., [Remember](https://codeforces.me/blog/entry/59228) that only the trusted
participants of the Div.3
Announcement of Codeforces Round 923 (Div. 3)
28.
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]...
29.
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...
3. The editorial was released quite fast after the round.
30.
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...
Why is it better than matrix multiplication? Besides it's $O(m^2\log(n))$
instead of $O(m^3\log(n
31.
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...
32.
Codeforces Global Round 14 Hi!
On [contest_time:1515], we will host [contest:1515]. **<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%;">**
This is the second 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:
- 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 i...
,2021-04-26] for Codeforces and Polygon platforms! - You, for being you! You
will have **3 hours, 1. [user:Radewoosh,2021-05-02] 2. [user:ksun48,2021-05-02] 3.
[user:Um_nik,2021-05-02] 4, You will have **3 hours** to solve **9 problems**. Scientific studies have
shown irrefutable
Announcement of Codeforces Global Round 14
33.
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]...
minutes to the round, but we should have announced it. 3. Editorial is posted:
https://codeforces.com, 1. [user:Nachia,2020-04-12] 2. [user:Riviclia,2020-04-12] 3.
[user:0.142857,2020-04-12] 4, 1. [user:tourist,2020-04-12] 2. [user:Radewoosh,2020-04-12] 3.
[user:tlwpdus,2020-04-12] 4, ], [user:Sad_reacts_only,2020-04-10], [user:TselmegKh,2020-04-10] - **Phase 3**
(Before few days to the round, ], [user:TselmegKh,2020-04-10] - **Phase 3** (Before few days to the round
Announcement of Codeforces Round 633 (Div. 1)
Announcement of Codeforces Round 633 (Div. 2)
34.
Codeforces Round 1134 (Div. 4) Привет, Codeforces!
Я рад пригласить вас на раунд [contest:2290], который пройдёт
[contest_time:2290]. После довольно долгого перерыва я снова взялся за
подготовку задач — на этот раз в паре с ИИ-агентами. Для меня это эксперимент
по возвращению в подготовку задач, и мы вместе посмотрим, насколько ИИ окажется
успешным напарником в этой деятельности. Жду на раунде и участников четвёртого
дивизиона, и всех, кто решит поучаствовать вне конкурса!
<div style="height:1em;"></div>
<spoiler summary="Почему я решил стать автором этого раунда (немного лирики и графоманства)">
Да, я прямо сейчас надиктовываю эти слова в телефон, чтобы позже облечь их в
форму перед вами.
Мы живём в удивительное время, когда очень многое меняется. Я программирую уже
больше 25 лет и на протяжении всего этого времени каждый день писал код, каждый
день решал задачи. Сначала это были больше задачи по программированию — они
настолько меня увлекали, что весь мир вращался вокруг этих задач, я был
пол...
was at the very end of a Div. 3 round. 99.5% of submissions start being judged
less than a minute, составило чуть больше трёх минут — да и то в самой концовке Div. 3. 99,5%
решений начинают, /7b76807d6cbeac0994fd894e56bafe8e90061651.jpg">
I recently celebrated my 3-year gym, Формат соревнования такой же, как в раундах Div. 3:
35.
Good Bye 2019 Снова привет, Codeforces!
Мы ради пригласить вас на <s>Mathforces</s> <s>Thinkforces</s> [Good Bye 2019](https://codeforces.me/contests/1270), который пройдет [contest_time:1270].
Немного информации о раунде:
<ul>
<li>Рейтинговый для всех участников!</li>
<li>3 часа!</li>
<li> <strong> Никаких подзадач! </strong> </li>
<li> В раунде будет <strong> интерактивная задача </strong>. Вы можете ознакомиться с руководством по интерактивным задачам [здесь](https://codeforces.me/blog/entry/45307)</li>
<li> Разбор будет опубликован сразу после окончания системного тестирования </li>
</ul>
Все задачи в этом раунде были приготовлены нами, [user:antontrygubO_o,2019-12-28] и [user:kefaa2,2019-12-28]. Мы долго работали над эти раундом и постарались сделать все задачи очень интересными. Надеемся, вам понравится раунд!
Мы бы хотели поблагодарить:
<ul>
<li> [user:arsijo,2019-12-24] за координацию раунда </li>
<li> [user:GandalfTheGrey,2019-12-24] за идею решени...
информации о раунде:
* Рейтинговый для всех участников!
* 3 часа! , Some information about the round:
* Rated for all participants!
* 3 hours!, Немного информации о раунде:
* Рейтинговый для всех участников!
* 3 часа!
Announcement of Good Bye 2019
36.
[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...
$xA’(x) = a_{1}x^{1} + 2a_{2}x^{2} + 3a_{3}x^{3} + … = \displaystyle\sum_{n
\ge 0}na_{n}x^{n}$., \ge 3}\frac{(n-1)!}{2n!}x^{n} = \frac{1}{2}\displaystyle\sum_{n \ge 3
}\frac{x^{n}}{n} = \frac{1}{2, definition, we have $A(x) = 1+x+x^{2}+x^{3}+...$. Does this series look
familiar?, partition $\\{1,2,...,n\\}$ into disjoint sets. For example, $b_{3}=5$,
because there are $5$ ways to, $-\ln (1-x) = x + \frac{x^2}{2} + \frac{x^3}{3} + ... = \displaystyle\sum_{n
\ge 1}\frac{x^{n}}{n}$, $. For example, $\\{3,1,1\\}$ is a partition of $5$ into $3$ parts. Note that
the order of elements, $= 1 + \displaystyle\sum_{n \ge 1}\frac{(-1)^{n-1}(1 \cdot 3 \cdot ... \cdot
(2n-3))}{2^{n}} \cdot, $= \displaystyle\sum_{n \ge 0}\frac{1}{2} \cdot \frac{-1}{2} \cdot \frac{-3}{2}
\cdot ... \cdot, $e^{x} = 1 + x + \frac{x^2}{2!} + \frac{x^3}{3!} + ... = \displaystyle\sum_{n
\ge 0}\frac{x^{n}}{n!}$, . 2 C. However, why stop here? Suppose the problem asks you to find $b_{n}$ for
$n \le3 \cdot 10^{5, For example, consider the Fibonacci sequence $f$ with the terms $0, 1, 1, 2, 3,
5, 8, …$. Then, $F
37.
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...
Out of these $10$ rounds, $4$ rounds are generally considered as good rounds, $3
$ rounds have
38.
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...
3. IP ban + blocked by the administrator, > MikeMirzayanov did make a comment about this api 3 years ago. However, it
seems like there was no
39.
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...
) Apologize to all participating universities (3) Promise to investigate
thoroughly and make the
40.
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...
). * I don't think that Div. 3 rounds are too dificult. Actually, they are
quite good for beginners, * I don't think that Div. 3 rounds are too dificult. Actually, they are quite
good for beginners, * 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
41.
Codeforces Raif Round 1 [Div. 1 + Div. 2] Hello, Codeforces!
Welcome to the [contest:1428] supported by Raiffeisenbank, that will start on [contest_time:1428]. **It will be a combined rated round for both divisions. Note that the start time is unusual.**
All problems were authored and prepared by [user:bensonlzl,2020-10-15], [user:oolimry,2020-10-15], [user:errorgorn,2020-10-15], [user:dvdg6566,2020-10-15], [user:shenxy13,2020-10-15].
[user:Ari,2020-10-15] gato to:
- [user:isaf27,2019-05-10] and [user:KAN,2020-10-15] for round coordination and help with preparation
- [user:zhangguangxuan99,2020-10-15] for being epic IOI trainer <3
- Our army of testers: [user:Ari,2020-10-15], [user:KAN,2020-10-15], [user:Monogon,2020-10-15], [user:Prabowo,2020-10-15], [user:SYY,2020-10-15], [user:SleepyShashwat,2020-10-15], [user:TeaTime,2020-10-15], [user:Tlatoani,2020-10-15], [user:_rs_,2020-10-15], [user:agul,2020-10-15], [user:cstuart,2020-10-15], [user:dantoh,2020-10-15], [user:jhkoh,2020-10-15], [user:kai824,2020-10-15]...
IOI trainer <3 - Our army of testers: [user:Ari,2020-10-15],
[user:KAN,2020-10-15], [user:Monogon, **UPD 3: First ACs and winners**, - 1-3 место = Беспроводная колонка, - [user:zhangguangxuan99,2020-10-15] for being epic IOI trainer <3, - [user:zhangguangxuan99,2020-10-15] за то, что он лучший IOI тренер <3, 3. [user:kczno1,2020-10-17]
Announcement of Codeforces Raif Round 1 (Div. 1 + Div. 2)
42.
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...
- GPT writes an obvious $O(N^3)$ DP. Works. - $O(N)$ greedy. Wrong, ofc. But
works. - $O(N)$ DP, The first contest lacked (1), while the re-contest failed at (2), (3), and (4).
43.
Codeforces Round #715 <p style="text-align: right;">
<i>
"Love" is a violent word.<br>
"I love that about you..." Doesn't that just mean "If that changed, I wouldn't love you anymore?"<br>
"Love" is a word that binds you.<br>
— Touko Nanami —
</i>
</p>
Hi Codeforces!
[user:Ari,2021-04-13] and I are pleased to invite you to participate in [contest:1508] and [contest:1509], which will be held at [contest_time:1508]. Each division will have **6 problems** and **2 hours and 15 minutes** to solve them.
Big thanks to the following people:
- Sir [user:antontrygubO_o,2021-04-13] for being an amazing coordinator; the round wouldn't happen without you ❤️
- [user:Scrubpai,2021-04-13], [user:1-gon,2021-04-13], [user:hugopm,2021-04-13], [user:tfg,2021-04-13], [user:dorijanlendvaj,2021-04-13], [user:xuanquang1999,2021-04-13], [user:300iq,2021-04-13], [user:Mike4235,2021-04-13], [user:Luminal,2021-04-13], [user:QuangBuiYT,2021-04-13], [user:jamienguyen,2021-04-13], [user:Pichu,2021-04-13], [user:ch...
- Div. 1: 1. [user:ecnerwala,2021-04-16] 2. [user:ksun48,2021-04-16] 3.
[user:tourist,2021-04-16, -04-16] 3. [user:tourist,2021-04-16] 4. [user:Radewoosh,2021-04-16] 5.
[user:maroonrk,2021-04-16, 1. [user:wannahavealife,2021-04-16] (solved all problems!) 2.
[user:traxex2,2021-04-16]3. [user
Announcement of Codeforces Round 715 (Div. 1)
Announcement of Codeforces Round 715 (Div. 2)
44.
Codeforces Round #589 (Div. 2) **다시 만나서 반가워요, 코드포스!** (Nice to see you again, Codeforces!)
I'm again happy to introduce you to [Codeforces Round #589 (Div. 2)](https://codeforces.me/contests/1228). Please look at following information for details:
- This contest will take place on [contest_time:1228].
- The round will be **rated for all Division 2 participants**.
- There are **6 problems** and you will have **2 hours** to solve them. Score distribution will be announced later.
The listed handles below are contributors. Thank you for all who listed!
- **Main author**: ~McDic,2019-09-26 — I made and prepared all problems.
- **Coordinator**: ~arsijo,2019-09-26 — He is my coordinator again.
- **Early phase testers**: ~dorijanlendvaj,2019-09-26 ~tfg,2019-09-26 ~Lawali,2019-09-26 — They are early phase testers. They mostly focused on test data.
- **Late phase testers**: ~_Kuroni_,2019-09-26 ~imbr92,2019-09-26 ~Redux,2019-09-26 ~Dr_Park,2019-09-26 ~Mahotsukai,2019-09-26 ~TOPWINDFLOWER,20...
appeared in another Div.1 contest. 3. Scoring distribution is
**500(A)-1000(B)-1250(C)-1750(D)-2250, solved that problem for a week. That problem will be appeared in another Div.1
contest.3. Scoring, 1. ~PositionZero,2019-09-29 2. ~supy_2,2019-09-29 3. ~hitman623,2019-09-29 4.
~lzqaq,2019-09-29
Announcement of Codeforces Round 589 (Div. 2)
45.
Testing Round 20 [Communication Problems] Hello Codeforces.
We are introducing a new type of problem which may appear in future Codeforces rounds, called Communication problems!
In these problems, your program would be ran twice, with different input and output formats between the runs. All variables stored in the memory will be lost between runs. However, the information given to you on the first run may be important to you for completing the second run correctly. Therefore, one of the key challenges in these types of problems would be to find a way to use the limited amount of output you are given in the first run to communicate information to the second run. Below shows a flowchart.
<center>
<br/>
<img src="https://i.ibb.co/JWMFpyfY/a-1.png" alt="2025-09-06-12-40-07" border="0" />
<br/>
Flowchart for communication problems
</center>
In these problems, time and memory limits for both runs would be kept separately. For example, if a problem has a time limit of $2$ seconds, you will only get the Time Limit E...
start on [contest_time:2168]. You will be given $1$ hour to solve $3$
run-twice problems. The, ]. You will be given $1$ hour to solve $3$ run-twice problems. The problems are
authored by [user:cry
Announcement of Testing Round 20 (Unrated, Communication Problems)
46.
Codeforces Round #638 (Div. 2) Hi Codeforces!
I’m thrilled to invite you to [contest:1348], which will take place on [contest_time:1348]. It's rated!
There will be **6 problems**, and you will have **2.5 hours** to solve them. All the problems are written and prepared by me.
Huge thank you to everyone who made this round possible:
- [user:adedalic,2020-04-28] for awesome coordination, guidance, advice, and more <3
- [user:dragonslayerintraining,2020-04-28] for extensive testing, feedback, and buffing tests
- [user:pikmike,2020-04-28], [user:walnutwaldo20,2020-04-28], [user:Benq,2020-04-28], [user:hocky,2020-04-28], [user:Geothermal,2020-04-28], [user:isaf27,2020-04-28], [user:McDic,2020-04-28], [user:bfs.07,2020-04-28], [user:iankury,2020-04-28], [user:songsinger,2020-04-28], [user:Agnimandur,2020-04-28], [user:dcshin,2020-04-28], [user:chrome,2020-04-28], [user:Rods,2020-04-29], [user:brian,2020-04-29], and [user:HAEtaRhoEta,2020-04-30] for taking the time to test and provide invaluable advice
...
round possible: - [user:adedalic,2020-04-28] for awesome coordination,
guidance, advice, and more <3, - [user:adedalic,2020-04-28] for awesome coordination, guidance, advice, and
more <3, 3. [user:NishimiyaShouko,2020-05-01], 3. [user:peti1234,2020-05-01]
Announcement of Codeforces Round 638 (Div. 2)
47.
Goodbye TheScrasse It is with a heavy heart that I announce the tragic loss of [user:TheScrasse,2025-03-31] to the codeforces community.
I have known [user:TheScrasse,2025-03-31] from coordinating pinely round 3. It was was one of the best rounds I had the pleasure of coordinating in recent memory. More recently, [user:TheScrasse,2025-03-31] has also been actively involved in coordinating some of the best rounds in codeforces recently.
Outside of problemsetting, [user:TheScrasse,2025-03-31] has also made a name for himself by writing educational blogs such as his [competitive programming roadmap](https://codeforces.me/blog/entry/111099) and [various](https://codeforces.me/blog/entry/113484) [other](https://codeforces.me/blog/entry/101271) [educational](https://codeforces.me/blog/entry/98624) [blogs](https://codeforces.me/blog/entry/91707) which enriched the knowledge of many within our community. His passion for sharing knowledge and nurturing talent has shaped the journeys of many aspiring pr...
codeforces community. I have known [user:TheScrasse,2025-03-31] from
coordinating pinely round3. It, I have known [user:TheScrasse,2025-03-31] from coordinating pinely round 3. It
was was one of the
48.
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...
will start at [contest_time:1842]. You will be given **9 problems** and **3
hours** to solve them. The, - _1st place: 1,024 TON_ - _2–3 places: 512 TON each_ - _4–7 places: 256 TON
each_ - _8–15, 1. [user:tourist,2023-06-25] 2. [user:maroonrk,2023-06-25] 3.
[user:hos.lyric,2023-06-25] 4, :1842]. You will be given **9 problems** and **3 hours** to solve them. The
round will be rated for
Announcement of CodeTON Round 5 (Div. 1 + Div. 2, Rated, Prizes!)
49.
Good Bye 2015 Hi everybody.
The last round of the 2015 will take place on the 30-th of December ([starting time](http://www.timeanddate.com/worldclock/fixedtime.html?day=30&month=12&year=2015&hour=18&min=5&sec=0&p1=166)).
The contest will last 3 hours.
It won't be a usual round.
Both divisions will compete together.
You will get 8 problems to solve in 3 hours.
Points will decrease slower than usually — otherwise you would get eps for solving a problem at the end.
Scoring will be announced just before a contest.
So will the speed of the points/minute loss.
My goal was to provide you a diverse problemset with interesting problems for all contestants.
During a contest you should consider reading not only the next problem but the few next ones.
You will get this round thanks to work of many people.
I am a problem setter.
[user:GlebsHP,2015-12-28] helps me with everything (a lot).
[user:AlexFetisov,2015-12-29], [user:johnasselta,2015-12-29] and [user:Zlobober,2015-12-29] are t...
contest will last 3 hours., in 3 hours. Points will decrease slower than usually — otherwise you would get
eps for, 1. [user:tourist,2015-12-30] 2. [user:Petr,2015-12-30] 3.
[user:Egor,2015-12-30] 4
Announcement of Good Bye 2015
50.
Meta Hacker Cup 2025 Schedule Meta Hacker Cup 2025
==================
Meta Hacker Cup is back! We’re excited to announce our schedule for our 2025 season, kicking off on October 10th!
- Practice Round: [Fri. October 10th, 10am Pacific (120 hours, 5 days)](https://www.timeanddate.com/worldclock/fixedtime.html?msg=Meta+Hacker+Cup+Practice+Round&iso=20251010T10&p1=234)*
- Round 1: [Sat. October 18th, 10am Pacific](https://www.timeanddate.com/worldclock/fixedtime.html?msg=Meta+Hacker+Cup+Round+1&iso=20251018T10&p1=234&ah=3) (3 hours)
- Round 2: [Sat. November 1st, 10am Pacific](https://www.timeanddate.com/worldclock/fixedtime.html?msg=Meta+Hacker+Cup+Round+2&iso=20251101T10&p1=234&ah=3) (3 hours)
- Round 3: [Sat. November 15th, 10am Pacific](https://www.timeanddate.com/worldclock/fixedtime.html?msg=Meta+Hacker+Cup+Round+3&iso=20251115T10&p1=234&ah=3) (3 hours)
- Finals: [Sat. December 13th, 6am Pacific](https://www.timeanddate.com/worldclock/fixedtime.html?msg=Meta+Hacker+Cup+Finals&iso=20251213T06&p1=234&ah=4...
+Hacker+Cup+Round+1&iso=20251018T10&p1=234&ah=3) (3 hours) - Round 2: [Sat.
November 1st, 10am Pacific, =3) (3 hours) - Round 2: [Sat. November 1st, 10am
Pacific](https://www.timeanddate.com/worldclock
51.
[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...
52.
Code Weekend #1 <div align="right" margin="20px">
<img align="right" src="/predownloaded/72/f1/72f1f31badb1452243838a0fc2553c04accff164.jpeg" alt="logo" width="300px" height="300px" margin="20px"/></div>
Всем привет!
Мы, команда [RGBTeam](https://clist.by/team/Romka,tourist,%E2%88%A83886090/) ([user:qwerty787788,2024-05-21], [user:Romka,2024-05-21] и [user:tourist,2024-05-21]) очень любим эвристические соревнования по программированию и очень грустим из-за того, что с каждым годом их становится всё меньше и меньше. И вот, как-то однажды мы подумали, что хватит это терпеть, и решили организовать своё соревнование :) Несмотря на то, что часто такие идеи остаются лишь идеями, в этот раз мы были полны решимости довести дело до конца и сейчас хотим пригласить вас всех поучаствовать в первом и, хочется надеяться, не последнем **[Code Weekend #1!](https://codeweekend.dev)**
Соревнование пройдёт в формате, довольно близком к ICFPC/Hashcode — будет дана одна задача и несколько десятков тестов к н...
three tests while Team B has the best result in the remaining seven, Team A
will receive 300/10*3, команды В, то команда А получит 300/10*3 = 90 TON, а команда В — 300/10*7 =
210 TON., - 1-е место — 400 TON - 2-е место — 300 TON - 3-е место — 200 TON - 4-е место
53.
Codeforces Global Round 8 <img src="/predownloaded/2e/e2/2ee2f0f95f05aae6ca6f47812dae2dc2aabf277b.png" style="width:200px; float:right; margin: 0 1em 1em 1em"/>
Hi!
On [contest_time:1368] we will host [contest:1368].
It is the second 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](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 are set by me. Thanks a lot to the coordin...
1. [user:ecnerwala,2020-06-19] 2. [user:tourist,2020-06-19] 3.
[user:Marcin_smu,2020-06-19] 4
Announcement of Codeforces Global Round 8
54.
VK Cup '22/23 — Отбор (Engine) и Codeforces Round #844 (Div. 1 + Div. 2) Привет!
[contest:1781] начнётся уже скоро: [contest_time:1781]. Это соревнование предназначено для тех, кто решил хотя бы 6 задач из 8 в квалификационном раунде [VK Cup 2022](https://codeforces.me/vkcup2022). Раунд будет рейтинговым.
Остальных приглашаем на открытый для всех [contest:1782], который начнётся в то же время и тоже будет рейтинговым.
Все задачи придуманы и подготовлены мной. Также этот раунд стал лучше благодаря [user:KAN,2023-01-13], [user:errorgorn,2023-01-13], [user:lperovskaya,2023-01-13], [user:dario2994,2023-01-15], [user:Monogon,2023-01-15], [user:Arpa,2023-01-15].
Участникам будет предложено 8 задач и 3 часа на их решение.
64 лучших участника закрытого отборочного раунда получат фирменные футболки VK Cup, а 16 лучших пройдут в финал и смогут побороться за призы 4-5 февраля очно в офисе VK или онлайн:
- 1-е место — 300 000 рублей;
- 2-е место — 250 000 рублей;
- 3-е место — 150 000 рублей;
- 4-е место — 100 000 рублей.
...
- 1-е место — 300 000 рублей; - 2-е место — 250 000 рублей; - 3-е место — 150, 1. [user:orzdevinwang,2023-01-24] 2. [user:noimi,2023-01-24] 3.
[user:Radewoosh,2023-01-24] 4, 1. [user:turmax,2023-01-24] 2. [user:Petr,2023-01-24] 3.
[user:never_giveup,2023-01-24] 4, You will be given 8 problems and 3 hours to solve them., Участникам будет предложено 8 задач и 3 часа на их решение.
Announcement of VK Cup 2022 - Отборочный раунд (Engine)
55.
Good Bye 2022: 2023 is NEAR "It's been a long time since I came here, and I've really been through a lot." Koxia muses as she chatted idly with Mahiru.
Now that the Winter Festival is approaching. Among the flash of fireworks, in the chimes of the New Year, what is waiting for them to encounter?
Armed with girlish courage, they stepped onwards.
Hello Codeforces!
We ([user:Nanako,2022-12-27], [user:m_99,2022-12-27], [user:huangxiaohua,2022-12-27], [user:SteamTurbine,2022-12-27], [user:triple__a,2022-12-27], [user:Nezzar,2022-12-27]) are very pleasured to invite you to take part in [contest:1770], which will take place in [contest_time:1770]!
This round consists of **8 tasks** waiting for you to solve in **150 minutes**, and will be rated for everyone!
On behalf of the author team, please allow me to express our sincere thanks to:
* [user:errorgorn,2022-12-27], for useful guidance and brilliant advice during the coordinati...
follows: - Ⓝ 1024 for the first place - Ⓝ 512 for the 2-3 places - Ⓝ 256 for
the 4-7 places, - Ⓝ 1024 for the first place - Ⓝ 512 for the 2-3 places - Ⓝ 256 for the 4-7
places - ... - Ⓝ 1, 1. [user:Benq,2022-12-30] 2. [user:maroonrk,2022-12-30] 3.
[user:Radewoosh,2022-12-30] 4
Announcement of Good Bye 2022: 2023 is NEAR
56.
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...
will have 3 hours to solve 8 problems (and 2 subtasks). If you want to lose
rating, then we encourage, 1. [user:Benq,2020-12-06] 2. [user:tourist,2020-12-06] 3.
[user:jiangly,2020-12-06] 4, You will have 3 hours to solve 8 problems (and 2 subtasks). If you want to lose
rating, then we
Announcement of Codeforces Global Round 12
57.
CodeTON Round 2 Hello, Codeforces!
A wonderful summer holiday! After College Entrance Examination, we are extremely delighted to invite you to our second round, [contest:1704], which will be held on [contest_time:1704]. **Note the unusual start time of the round.**You are given **8 problems** and **2.5 hours** to solve them.
All problems were written and prepared by [user:Cirno_9baka,2022-07-23], [user:CoupDeGrace,2022-07-23], [user:Heltion,2022-07-23], [user:ODT,2022-07-23], [user:Yakumo_Ran,2022-07-23], [user:farmerj,2022-07-23], [user:flowerletter,2022-07-26], [user:izlyforever,2022-07-23], [user:kuangbin,2022-07-23], [user:mejiamejia,2022-07-23], [user:ugly2333,2022-07-23] and me.
Task statements and editorials will also be available in Chinese (Simplified) and Chinese (Traditional) after the contest.
We are sincerely thankful for the help provided by:
- [user:3.141592653,2022-07-25], [user:emorgan5289,2022-07-25], [user:errorgorn,2022-07-25], [user:SevenDawns,2022-07-25], [user:pr...
* _1st place: 1,024 TON_ * _2–3 places: 512 TON each_ * _4–7 places: 256 TON
each_ * _8–15, 1. [user:tourist,2022-08-01] 2. [user:jiangly,2022-08-01] 3.
[user:ksun48,2022-08-01] 4
Announcement of CodeTON Round 2 (Div. 1 + Div. 2, Rated, Prizes!)
58.
It's Happening! Meta Hacker Cup 2023 Schedule Meta Hacker Cup 2023 Schedule
==================
Meta Hacker Cup is back! We’re excited to announce our schedule for our 2023 season, kicking off on **September 22nd**!
- Practice Round: [Fri. Sept 22nd, 10am Pacific — Mon. Sep. 25th, 10am Pacific](https://www.timeanddate.com/worldclock/fixedtime.html?msg=Meta+Hacker+Cup+2023+Practice+Round&iso=20230922T10&p1=234) (72 hours)*
- Round 1: [Sat. Oct. 7th, 10am — 1pm Pacific](https://www.timeanddate.com/worldclock/fixedtime.html?msg=Meta+Hacker+Cup+2023+Round+1&iso=20231007T10&p1=234&ah=3) (3 hours)
- Round 2: [Sat. Oct. 21th, 10am — 1pm Pacific](https://www.timeanddate.com/worldclock/fixedtime.html?msg=Meta+Hacker+Cup+2023+Round+2&iso=20231021T10&p1=234&ah=3) (3 hours)
- Round 3: [Sat. Nov. 4th, 10am — 1pm Pacific](https://www.timeanddate.com/worldclock/fixedtime.html?msg=Meta+Hacker+Cup+2023+Round+3&iso=20231104T10&p1=234&ah=3) (3 hours)
- Finals: [Sat. Dec. 9th, 6am — 10am Pacific](https://www.timeanddate.com/worldclock...
+Hacker+Cup+2023+Round+1&iso=20231007T10&p1=234&ah=3) (3 hours) - Round 2:
[Sat. Oct. 21th, 10am — 1pm, /fixedtime.html?msg=Meta+Hacker+Cup+2023+Round+1&iso=20231007T10&p1=234&ah=3) (3
hours) - Round 2
59.
I challenge you to a duel, Errichto (UPD: Saturday 11am PT) 
Dear [user:Errichto,2020-09-17],
In case you are unaware, you beat me by 3 seconds of penalty on [Facebook Hacker Cup Round 3](https://www.facebook.com/codingcompetitions/hacker-cup/2020/round-3/scoreboard). Upon deep reflection, I decided that I will not accept this lying down. In order to avenge this notorious defeat, I am challenging you to a 1v1 duel.
**Rules:**
1. Duel will be held as a lockout competition and will be televised live
2. Lockout problems will be chosen by a neutral problem setter
3. Loser must change their main to honor the winner. See below for details
~~~~~
// your template if you lose
#define stevenkplus main
int stevenkplus() {
// your code here
}
~~~~~
---
**Update:**
We're doing it this Saturday! https://www.timeanddate.com/worldclock/fixedtime.html?msg=Errichto+vs+Stevenkplus&iso=20200926T11&p1=224&ah=1
Will be broadcast live at https://www.twitch...
[user:Errichto,2020-09-17], In case you are unaware, you beat me by 3 seconds
of penalty on [Facebook, be chosen by a neutral problem setter 3. Loser must change their main to honor
the winner. See, In case you are unaware, you beat me by 3 seconds of penalty on [Facebook
Hacker Cup Round3](https
60.
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 ...
Then we can get the ODE of $P(H)$ : $$P(H)H'^3=P(H)'((m-n+1) H'^2 - H''H) +
P(H)''H'H$$, which is, ####[Chain Reaction](http://uoj.ac/problem/50) in UOJ Round 3 By
[user:vfleaking,2020-04-23], )'}{H'}, P''(H)=\frac{P(H)''H'-P(H)'H''}{H'^3}$$.
61.
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...
![ ](/predownloaded/4a/0c
62.
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">
.
All problems were written and prepared by [user:CoupDeGrace,2021-07-02], [user:kuangbin,2021-07-02], [user:mejiamejia,2021-07-02], [user:Heltion,2021-07-02], [user:Melacau,2021-07-02], [user:Nanako,2021-07-02], [user:GOATWU,2021-07-02], [user:Cirno_9baka,2021-07-02], [user:Suiseiseki,2021-07-02], [user:ODT,2021-07-02], [user:box,2021-07-03], [user:Ynoi,2021-07-02], [user:syh0313,2021-07-02], [user:wh0816,2021-07-02] and me.
And thanks to [user:QAQAutoMaton,2021-07-02], [user:gamegame,2021-07-02], [user:starusc,2021-07-02], [user:interesting...
3.[user:RBurgundy,2021-07-12], 3.[user:xtqqwq,2021-07-12]
Announcement of Codeforces Round 732 (Div. 1)
Announcement of Codeforces Round 732 (Div. 2)
64.
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...
Trick — Geometry being useful](https://codeforces.me/blog/entry/63823) -
[Lecture #3 — Exchange, ) - [AlgorithmsThread Ep 3: Segment Trees (+ hard ST
Problem)](https://codeforces.me/blog/entry/79284) - [A, ) - [PART 3 TOP anime characters from CODEFORCES
2021!](https://codeforces.me/blog/entry/88431
65.
Codeforces Round #736 Hello, Westeros!
I'm glad to invite you to [contest:1548] and [contest:1549], which will be held on [contest_time:1548].
The round will be rated for both divisions. Each division will have 5-7 problems and 2 hours and 15 minutes to solve them. There **will not** be an interactive problem, so yay!!!
This round would not have been possible without the following individuals:
1. [user:Aleks5d,2021-07-30], for awesome coordination of my round.
2. [user:Benq,2021-07-30], for extensive testing and contributions throughout the round, especially for [problem:1548E].
3. [user:1-gon,2021-07-30], for discussing problems and statements with me for hours on Discord.
4. [user:amgfrthsp,2021-08-01], for translating statements into Russian.
5. [user:MikeMirzayanov,2021-08-01], for Codeforces and Polygon.
#### 32 testers
The round had a total of 32 testers. I tried to get a "rainbow" of testers to help guarantee a most balanced round. Thank to you to each and every one of them!
- ...
extensive testing and contributions throughout the round, especially for
[problem:1548E].3. [user, testing and contributions throughout the round, especially for [problem:1548E].
3. [user:1-gon,2021, #### Div2 1. [user:dingdingsb,2021-08-02] 2. [user:pAFOstd,2021-08-02] 3.
[user:black_trees,2021, ], congratulations on the AK! 3. [user:yhx-12243,2021-08-02] 4.
[user:heno239,2021-08-02] 5, ]: [user:speedforces,2021-08-02] at 4 minutes! - [problem:1548A]:
[user:tourist,2021-08-02] at3
Announcement of Codeforces Round 736 (Div. 1)
Announcement of Codeforces Round 736 (Div. 2)
66.
Some thoughts on recent discussions Hello everyone!
I finally decided to write some response/explain my view after all recent discussions of my problems and rounds I coordinate, and I would like to make a few points.
**1.** After some recent contests there were a lot of comments saying that Data structure problems should appear as easy problems (say, D2A-D2D in a Div2 of $6$ problems)
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...
**3.** While I understand that many people really like these topics and want
them to appear as, To begin with, this is not true. All topics appear! I went through last $3$
Div2 contests I
67.
Codeforces Round #580 
Hello again, Codeforces!
I am glad to invite you to [Codeforces Round 580](https://codeforces.me/contests/1205,1206), which will take place on [contest_time:1205]. Round will be rated for both divisions.
All problems in this round were created and prepared by me, [user:antontrygubO_o,2019-08-16]. I tried to make them interesting and hope that you will enjoy them!
A lot of thanks to [user:arsijo,2019-06-29] for the excellent coordination of the round, [user:kefaa2,2019-06-29], [user:gepardo,2019-06-29], [user:danya.smelskiy,2019-08-16], [user:re_eVVorld,2019-08-16], [user:Xellos,2019-08-16], [user:GandalfTheGrey,2019-06-29], [user:prof.PVH,2019-08-16], [user:KAN,2019-08-16] for the testing and valuable comments, and to Mike [user:MikeMirzayanov,2019-06-29] Mirzayanov for the amazing platforms Codeforces и Polygon.
Participants in each division will be offered 6 problems and 2 hours 10 minutes to solve them. As usual, I **strongly** ...
**3.** [user:mnbvmar,2019-08-18], **3.** [user:ujrepacul,2019-08-18]
Announcement of Codeforces Round 580 (Div. 1)
Announcement of Codeforces Round 580 (Div. 2)
68.
CPIdeas: use AI to generate competitive programming ideas Hello Codeforces!
Being asked to propose competitive programming questions is pretty haunting. When you're out of fresh ideas, I used to do one of the two things. One, is to search in the old pile of problems, hoping to find some room of modifications and improvements. The second, is to come up with random words, like "chessboard inversion counting", and hopefully resemble interesting problems from them.
This process is pretty boring, so I have been trying to use machine learning to generate ideas and even complete competitive programming problems. The result is *CPIdeas*! Check it out here: https://fjzzq2002.github.io/cpideas/.
*How was it made?* I collected problems from AtCoder (ABC, ARC, AGC) and used these problems to fine-tune GPT-3, the OpenAI model. It's quite tricky to get things right though and it's still far from perfect.
*How should I use it?* Look through these ideas. Scroll down. Be tolerant and creative. That's it.
*How should I use these ideas?* For the ...
fine-tune GPT-3, the OpenAI model. It's quite tricky to get things right
though and it's still far from
69.
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...
Div. 2 and Div. 3 only contests)? Hard to tell, but in many cases it's hard to
believe that they
70.
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:...
**UPD 3:** Congratulations to the winners! :D, 1. [user:I_Love_Convex_Hull,2020-11-25] 2. [user:221,2020-11-25] 3.
[user:afterall,2020-11-25] 4, 1. [user:noimi,2020-11-25] 2. [user:I_Love_Convex_Hull,2020-11-25] 3.
[user:zhouzhendong,2020-11
Announcement of Codeforces Round 685 (Div. 2)
71.
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], [...
will take place on [contest_time:1466]. You will have **3 hours** to solve **9
problems**. The round, /contests/1466), which will take place on [contest_time:1466]. You will have **3
hours** to solve **9, 1. [user:tourist,2020-12-30] 2. [user:maroonrk,2020-12-30] 3.
[user:Benq,2020-12-30] 4
Announcement of Good Bye 2020
72.
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...
- Create some new accounts and take the win of Div.2/3/4 easily.
73.
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 ...
(together with Wunder Fund) * Sponsored rounds AIM Tech Round 2 and 3
(together with AIM Tech, (совместно с Wunder Fund) * спонсорские раунды AIM Tech Round 2 и 3 (совместно
с AIM Tech, necessary to me: at this moment are no deadlines, do not press the obligation.
Just now, at3 am, I, , в 3 часа ночи я не могу оторваться от того, чтобы сделать что-то новенькое.
Хочется воспользоваться
74.
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...
1. [user:boboniu,2020-08-16] 2. [user:maroonrk,2020-08-16] 3.
[user:ecnerwala,2020-08-16] 4, You will be given **3 hours** to solve **9 problems**. The score distribution
will be announced at
Announcement of Codeforces Global Round 10
75.
Codeforces Round #556 Hi Codeforces!
I'm so happy to invite you to [contest:1149] and [contest:1150]! The rounds will be held on [contest_time:1149]. The round will be rated for both divisions. (At least that's what I've been told!)
The problems were written and prepared by me.
Thanks to [user:300iq,2019-04-27] for the round coordination, and to [user:Radewoosh,2019-04-27] for his invaluable help with choosing the problemset and testing the problems!
Also, I want to give a shout-out to [user:MikeMirzayanov,2019-04-27] for his amazing Codeforces and Polygon platforms!
_My browser's tab bar, right now._
You'll work on **5** problems, and you will have **2** hours to solve them. The scoring distribution will be revealed closer to the round.
~~~~~
def get_end_remark():
return random.choice([
"Wish everyone high ratings!",
"Good luck!",
"Have fun!",
"Please, read all the problems!"])
~~~...
look at the [editorial](https://codeforces.me/blog/entry/66783)! --- **UPD 3
:** We finished the, **UPD 3:** We finished the system tests. Congratulations to the winners!, 1. [user:Um_nik,2019-04-29] 2. [user:Vn_nV,2019-04-29] 3.
[user:maroonrk,2019-04-29] 4, 1. [user:jiangly,2019-04-29] 2. [user:Simplicity,2019-04-29] 3.
[user:Netherdrake,2019-04-29] 4
Announcement of Codeforces Round 556 (Div. 1)
Announcement of Codeforces Round 556 (Div. 2)
76.
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...
://www.timeanddate.com/worldclock/fixedtime.html?msg=Errichto+Stream+%231&iso=20180929T10&p1=262&ah=
3)) here: [https, from Polish olympiad. 3. Post-contest stream, assuming I'm a setter or tester
of a contest, **--- UPDATE 3 ---**, 1. Solving random easy-medium problems. 2. Solving old problems from Polish
olympiad.3. Post
77.
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...
1. [user:maroonrk,2025-12-23] 2. [user:ecnerwala,2025-12-23] 3.
[user:turmax,2025-12-23] 4, Update 3: editorial — https://codeforces.me/blog/entry/149944, ]. You will be given **9 problems** and **3 hours** to solve them. **At least
one problem will be
Announcement of Hello 2026
78.
Filter for Past Contests Hello, Codeforces.
It's scary to think about it: there are already almost 1800 rounds in the list of past contests!
I started adding a filter to the table with these rounds. Right now, you can already filter by some of the types of rounds. Here's what I've supported:
* Div. 1
* Div. 2
* Div. 3
* Div. 4
* Div. 1 + Div. 2
* Educational Rounds
* CodeTON Rounds
* Globals
* Kotlin Rounds
* VK Cup Rounds
* Long Rounds
* April Fools
Of course, I might have forgotten some types of contests. Please remind me if I missed anything. We should pay attention only to types where we have conducted several contests of that type.
I plan to expand this filter by adding more settings. Perhaps it would be good to filter by the condition "has at least one submission in this contest" (or those for which no submissions at all). What do you think?
What else would you suggest adding?
P.S. Here is how it looks on the website (some intermediate version, some options may be missing)...
* Div. 1 * Div. 2 * Div. 3 * Div. 4 * Div. 1 + Div. 2 * Educational Rounds *
CodeTON Rounds
79.
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-...
1. [user:cnnfls_csy,2023-07-11] 2. [user:tourist,2023-07-11] 3.
[user:-0.5,2023-07-11] 4, You will be given **8 problems** and **3 hours** to solve them. One of the
problems is divided into
Announcement of Codeforces Round 884 (Div. 1 + Div. 2)
80.
Analyzing how good O1-Mini actually is I would like to thank [user:redpanda,2024-09-13] and [user:qmk,2024-09-13] for their great help in writing and reviewing this blog. They are also the users who queried O1-mini for all the following problems.
We tried O1-mini on several problems, from a variety of sources. Let's list the results first.
**Note** :
- Some of the WA verdicts here actually means that the AI just "stopped thinking" which means the AI thought for a long enough time without any useable results so it ran into an error.
- All ratings mentioned are Codeforces ratings, the Atcoder ratings have been converted to codeforces rating. To measure the approximate codeforces rating of the atcoder problems mentioned here, you can use https://kenkoooo.com/atcoder/#/table/ + https://silverfoxxxy.github.io/rating-converter.
<h4> D2ABs </h4>
- [problem:1762B] [<a class="rated-user user-gray" title="Gray">1100</a>] <a class="rated-user user-green" title="Green AC">AC</a>
- [problem:1998B] [<a class="rated-...
81.
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 ...
3. [user:Endagorion, 2015-02-16], 3. [user:zclimber, 2015-02-16]
Announcement of Codeforces Round 292 (Div. 2)
Announcement of Codeforces Round 292 (Div. 1)
82.
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...
#### 3. Get rid of those includes!, **e.g.** __builtin_popcount(14) = 3 because 14 is '... **111** 0' and has three
1-bits., Output: `5 3`, deque>> d; d = {{{3, 4}, {5, 6}}, {{1, 2}, {3, 4}}}; for (auto i: d, int main() { cout << sum(5, 7, 2, 2) + sum(3, 4); /* prints "23" */ } ~~~~~, tuple t; t = {3, 4, 'f'}; cout << get<2>(t) << '\n'; ~~~~~, ~~~~~ 8 3 2 1 1 ~~~~~, ~~~~~ auto f = [] (int a, int b) -> int { return a + b; }; cout << f(1, 2); //
prints "3" ~~~~~, ~~~~~ pair > p; // ... p = {3, {'a', 8ll}}; ~~~~~, ~~~~~ pair p; // ... p = make_pair(3, 4); ~~~~~, ~~~~~ pair p; // ... p = {3, 4}; ~~~~~, ~~~~~ pair p; vector v; // ... p = make_pair(3, 4); v.push_back(4); v.push_back, ~~~~~ pair p; vector v; // ... p = {3, 4}; v = {4, 5}; ~~~~~, ~~~~~ set s = {8, 2, 3, 1}; for (auto it: s) cout << it << ' '; // prints "1 2 3
8" ~~~~~, ~~~~~ set s = {8, 2, 3, 1}; for (set::iterator it = s.begin(); it != s.end();
++it, ~~~~~ tuple t (3, 4, 5, 'g'); int a, b; tie(b, ignore, a, ignore) = t, ~~~~~ v: w: 1 2 3 4 ~~~~~, ~~~~~ vector v = {1, 2, 3, 4}; vector w = move(v);, ~~~~~ vector v = {3, 1, 2, 1, 8}; sort(begin(v), end(v), [] (int a, int b) {
return a > b, ~~~~~ vector v = {8, 2, 3, 1}; for (auto &it: v) it *= 2; for (auto it: v) cout
83.
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...
1. [user:bmerry,2015-04-26] 2. [user:Egor,2015-04-26] 3. [user:Petr,2015-04-26]
4
Announcement of Codeforces Round 300
84.
Let's stop making anti-GPT problems? There seems to be a general trend of problem-setters going out of their way to ensure that their problems aren't solvable by AI. This obviously restricts the meta to the kinds of problems that AI is bad at: which, in practice seem to primarily be ad-hoc problems that require multiple observations. This approach doesn't always (and in my opinion, rarely) yield the most enjoyable/natural problems to humans.
The only motivation for this tendency is to ensure that CF ratings and leaderboards in contests retain some meaning.
What I want to express from this blog:
**I think we should stop intentionally making anti-GPT problems and optimize problem-setting for human enjoyment, with a complete disregard for how AI models (and people using said models to cheat) perform on these problems.**
I will try to explain why I think so below.
There are only two ways (distinct in their relevance to CP) in which AI gets better in the future:
<ol>
<li>
It becomes smarter than humans a...
engage in this activity for jobs cheat en masse using AI initially. 2. Ratings
lose value.3, initially. 2. Ratings lose value. 3. Organizations stop caring about ratings
when hiring. 4
85.
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...
1. [user:maroonrk,2020-09-09] 2. [user:ainta,2020-09-09] 3.
[user:ko_osaga,2020-09-09] 4, 1. [user:minhchauxinhdep1234,2020-09-09] 2. [user:potato167,2020-09-09] 3.
[user:guangmutianwang
Announcement of Codeforces Round 668 (Div. 1)
Announcement of Codeforces Round 668 (Div. 2)
86.
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...
1. [user:chokottodake,2020-09-01] 2. [user:1092515504,2020-09-01] 3.
[user:mickey639,2020-09-01, 1. [user:tourist,2020-09-01] 2. [user:Radewoosh,2020-09-01] 3.
[user:aid,2020-09-01] 4
Announcement of Codeforces Round 666 (Div. 1)
Announcement of Codeforces Round 666 (Div. 2)
87.
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...
1. [user:tourist,2018-12-30] 2. [user:MakeRussiaGreatAgain,2018-12-30] 3.
[user:Um_nik,2018-12-30, UPD: The top 3 participants eligible for ICPC 2019/20 season [can win a great
prize](https
Announcement of Good Bye 2018
88.
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...
Pinely Round 3 (Div. 1 + Div. 2), will start on [contest_time:1909]. You will be given **9 problems** and **3
hours** to solve them. One, 1. [user:zh0ukangyang,2023-12-23] 2. [user:maroonrk,2023-12-23] 3.
[user:jiangly,2023-12-23] 4
Announcement of Pinely Round 3 (Div. 1 + Div. 2)
89.
Testing Round #16 (Unrated) Hello, Codeforces!
Paul [user:stingray,2020-05-07] Komkoff is my hero! He found some issues in the network configuration. The most important of them was about flow control auto-negotiation: in case of our network routers the number of dropped packets reduced in times. Thanks!
To be sure that it really helped to fix the issue I ask you to take part in [contest:1351]. It will start on [contest_time:1351]. Probably, it will contain 3 easy problems which are not prepared for now (I'll use some old problems, I think). The only reason to host the round is to check the system under a load. It will be an unrated round.
I am upset about what happened with the last round. Once again, I apologize to both the participants and the writer, coordinator and testers. I wrote [user:Monogon,2020-05-07] about the incident and apologized.
I am sure that the only way to **absolutely** prevent unrated rounds is just don't host them. And this is not my way. The most popular rounds were hosted with...
start on [contest_time:1351]. Probably, it will contain 3 easy problems which
are not prepared for
Announcement of Testing Round 16 (Unrated)
90.
Как провести Div 1 или Div 2 раунд на Codeforces Как вы наверняка знаете, большинство раундов на Codeforces созданы активными участниками нашего сообщества. Мы очень благодарны талантливым авторам со всего мира за то, что они знакомят десятки тысяч участников наших соревнований со своими идеями, предоставляя возможность учиться и получать удовольствие от решения задач как новичкам, так и лучшим в мире спортивным программистам. Этот пост расскажет, как стать автором задач и провести раунд на Codeforces.
## Кто может стать автором
Обычно авторами наиболее интересных и качественных задач являются опытные участники. Так как наши ресурсы по обработке предложений задач ограничены, то с 1-го января 2021, для предложения своих задач вы должны удовлетворять следующим условиям:
- рейтинг — 2100 или выше, или
- уже был автором.
В будущем ограничения могут быть изменены, пост будет обновлен. Если вы уже открыли ваш раунд для ревью, то он будет рассмотрен независимо от действующих ограничений, не волнуйтесь.
Если вы не подхо...
single problems. Also, we don't consider proposals for Div 3/4 or Educational
rounds., рассматриваем предложения отдельных задач. Также мы не рассматриваем
предложения Div3/4 и образовательных
91.
Thank you for participating in the AtCoder World Tour Finals 2022 English (translated by GPT-4):
To AWTF2022 Contestants,
Thank you so much for participating in the contest. I apologize for not being able to communicate much due to my inability to speak English. Did you enjoy the contest event? I look forward to seeing you again in Japan.
To Everyone,
With the conclusion of the topcoder open (TCO) and Google Code Jam (GCJ), and the decision to hold Meta Hacker Cup 2023 online, the only on-site contest in 2023 where anyone from around the world can participate and truly determine the world's best is the AtCoder World Tour Finals.
I have participated in the topcoder open finals 5 times (Marathon division) and in the Google Code Jam finals once. Both tournaments were outstanding. From my experience as a competitive programmer, participating in these competitions has been significant, and I believe that this culture must continue.
The AtCoder World Tour Finals is a budding event. Compared to TCO and GCJ, it has a shorter history, and th...
92.
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...
the series, Round 5, is coming today in less than 3 hours!, **UPD3:** New dates of Rounds 3-5 (and hence my streams) have been announced!, **UPD4:** Just a friendly reminder: Round 3 ends in less than 24 hours, and
thus my next stream is, **UPD5:** Round 3 stream: [link](https://www.youtube.com/watch?v=01fGS_4wuGc).
Round 4 stream is, 3 has not started yet — therefore, the next streams have to be postponed.
Follow this blog
93.
[Tutorial] Пересечение матроидов простыми словами **[This article is also available in [English](https://codeforces.me/blog/entry/69287?locale=en)]**
Привет, CodeForces.
Я думаю, что матроиды — это прекрасные и очень мощные структуры, однако, не настолько хорошо известные в спортивном программировании.
Я познакомился с матроидами на Зимних Петрозаводских Сборах 2019. Там была задача, которая очевидно не решалась обычными способами, известными мне тогда. Разбор этой задачи состоял буквально из трех слов “just matroid intersection”. Тогда мне потребовалось больше двух дней дорешивания чтоб найти всю необходимую информацию и детали и написать решение, которое получает Accepted. И намного больше времени мне потребовалось чтобы понять почему это действительно работает и как именно это работает. (Я до сих пор сомневаюсь в некоторых деталях.)
Конечно, совершенно не трудно нагуглить все необходимые определения и статьи, связанные с этой темой, но мне кажется, что все они больше сконцентрированы на математической теории, с...
of yellow area ($C$ is $(3-5-7-8-6-4)$). Now, there are two ways to exchange
edges to complete $C, границу желтой области ($C$ — это $(3-5-7-8-6-4)$). Теперь, существует два
способа замены ребер, $ following exchange edges in $D_M (S)$. In example above we can also obtain
the same circuit $(2-3-5-4, $ следуя ребрам замен в $D_M (S)$. В примере выше мы также можем получить тот
же цикл $(2-3-5-4, 1. Exchange $(4,5)$ to $(1,5)$ 2. Exchange $(4,8)$ to $(8,9)$ 3. Exchange
$(1,2)$ to $(2,3)$ 4, 1. Заменить $(4,5)$ на $(1,5)$ 2. Заменить $(4,8)$ на $(8,9)$ 3. Заменить
$(1,2)$ на $(2,3)$ 4, 1. Отсортируем элементы носителя матроида по увеличению веса 2. Начнем с
пустого множества $S$3, Of course, we are not speaking about arbitrary classifications. These $3$
properties must hold for, There are only two colorful spanning trees of this graph ($S = \\{(1, 2), (2, 3
), (3, 4)\\}$ and $T, We can exchange $(1, 2)$ to $(2, 3)$, symmetrically we can exchange $(5, 6)$ to
$(3, 5)$. However, const int SOURCE = -1; const int NOT_VISITED = -2; const int NOT_FOUND = -3;, Конечно, мы не рассматриваем произвольные классификации. Эти $3$ свойства
должны выполняться для, Можно заменить $(1, 2)$ на $(2, 3)$, симметрично можно заменить $(5, 6)$ на $(3
, 5)$. Однако нельзя, Существует только для цветных остовных дерева в этом графе ($S = \\{(1, 2), (2,
3), (3, 4)\\}$ и $T
94.
VK Cup 2017 Round 3 + Codeforces Round #412 -- Tutorial Here is the tutorial of VK Cup 2017 Round 3 and Codeforces Round #412. Enjoy!
<spoiler summary="Is it rated?">
[tutorial:807A]
<spoiler summary="Code">
~~~~~
n = int(input())
results = []
for i in range(n):
results.append(list(map(int, input().split())))
for r in results:
if r[0] != r[1]:
print("rated")
exit()
for i in range(n):
for j in range(i):
if results[i][0] > results[j][0]:
print("unrated")
exit()
print("maybe")
~~~~~
</spoiler>
</spoiler>
<spoiler summary="T-Shirt Hunt">
[tutorial:807B]
<spoiler summary="More efficient code">
~~~~~
#include <bits/stdc++.h>
using namespace std;
int main() {
int p, x, y;
cin >> p >> x >> y;
for (int s = y; ; s++) {
if (s % 50 != x % 50) {
continue;
}
bool me = false;
int i = s / 50 % 475;
for (int j = 0; j < 25; j++) {
i = (i * 96 + 42) % 475;
if (i + 26 == p) {
me = true;
...
VK Cup 2017 Round 3 + Codeforces Round #412 -- Tutorial, VK Cup 2017 Round 3 + Codeforces Round #412 -- разбор задач, Here is the tutorial of VK Cup 2017 Round 3 and Codeforces Round #412. Enjoy!, Разбор задач VK Cup 2017 Round 3 и Codeforces Round #412 доступен на русском и
английском языках
95.
Codeforces Round #742 (Div. 2) Hey there Codeforces!
[user:flamestorm,2021-09-01] and I are glad to invite you to our first-ever Codeforces round, [contest:1567], which will be held on [contest_time:1567]. **This round will be rated for participants with rating lower than 2100**.
Special shoutouts to:
- [user:adedalic,2021-09-01], for his fantastic coordination and translation of the round!
- [user:MikeMirzayanov,2021-09-01], for creating the amazing Codeforces and Polygon platforms!
- [user:dorijanlendvaj,2021-09-01], [user:awoo,2021-09-01], [user:YouKn0wWho,2021-09-01], [user:PurpleCrayon,2021-09-01], [user:shxgun,2021-09-01], [user:ijxjdjd,2021-09-01], [user:stefdasca,2021-09-01], [user:epicxtroll,2021-09-01], [user:Aaeria,2021-09-01], [user:Eyed,2021-09-01], [user:rkm62,2021-09-01], [user:Shinchan01,2021-09-02], [user:BRCode,2021-09-02], [user:namanbansal013,2021-09-05], [user:Urvuk3,2021-09-01], [user:1_2_oatmeal,2021-09-01], [user:richy6,2021-09-01], and [user:tdpencil,2021-09-01] for testing th...
3. [user:LayCurse,2021-09-05], 3. [user:NOOB228,2021-09-05]
Announcement of Codeforces Round 742 (Div. 2)
96.
Petition to Display Color of Max Rating In Codeforces, the nickname color depends on the current rating.
I propose to use the historical max rating instead.
The nickname color is not only a bragging right, but also a good heuristic when choosing which blogs to open in "Recent actions", and which posts to read when scrolling through many comments. Here's why I think that we should use max rating:
1. It's less random.
2. Current rating is nowadays more affected by AI cheaters.
3. There will be less incentive to create a second account when you're afraid to drop below some threshold.
4. It will make recognizing others easier (because the color won't change often).
5. I don't want to be orange.
rating is nowadays more affected by AI cheaters. 3. There will be less
incentive to create a second, 1. It's less random. 2. Current rating is nowadays more affected by AI cheaters.
3. There will be
97.
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 ...
1. [user:maroonrk,2021-03-01] 2. [user:DmitryGrigorev,2021-03-01] 3.
[user:Petr,2021-03-01] 4, You will have 3 hours to solve 9 problems. We encourage you to read all the
problemsand
Announcement of Codeforces Global Round 13
98.
VK Cup 2021 — Финал (Engine) и Codeforces Round #740 И снова привет!
[contest:1563] начнётся совсем скоро: в [contest_time:1563]. В финале примут участие [32 лучших участника отборочного раунда](https://codeforces.me/contest/1544/standings), которые разыграют между собой солидные призы:
- 1-е место — 300 000 рублей;
- 2-е — 250 000;
- 3-е — 150 000;
- 4-е — 100 000;
- 5-е — 80 000;
- 6-е — 30 000;
- и 7-е — 20 000.
Следить за ходом соревнования и болеть за любимых участников можно будет [по этой ссылке](https://codeforces.me/spectator/ranklist/2488e608fc1223994a45bca34b4a2c51). Также смотрите прямую трансляцию [в сообществе Команды ВКонтакте](https://vk.com/tech) (нет, спойлеров к задачам финала не будет).
Всех остальных, кроме финалистов VK Cup 2021, мы приглашаем на [contest:1558] и [contest:1561], которые начнутся на пару дней позже: во [contest_time:1558]. Раунды будут рейтинговыми для всех.
Все задачи были придуманы и подготовлены мной. Большое спасибо всем, без кого этот раунд не смог бы состояться: [user:KAN...
- 1-е место — 300 000 рублей; - 2-е — 250 000; - 3-е — 150 000; - 4-е —
100 000; - 5-е — 80 000, 1. [user:never_giveup,2021-08-22] 2. [user:Maksim1744,2021-08-22] 3.
[user:Um_nik,2021-08-22] 4
Announcement of VK Cup 2021 - Final (Engine)
99.
Hello 2020 **새해 복 많이 받으세요, 코드포스!** (Happy new year, Codeforces!)
Welcome to the first Codeforces Round of the new decade, [Hello 2020](https://codeforces.me/contests/1284)! The round will be held on [Jan/04/2020 15:05 MSK](https://www.timeanddate.com/worldclock/fixedtime.html?day=4&month=1&year=2020&hour=15&min=5&sec=0&p1=166).
Some information about the round:
* Div 1, 2 combined
* 2.5 hours!
* **7 problems!**
* No subtasks!
* Score distribution: 500-1250-1750-2500-2750-4000-4000
* Yes, it is rated!
This round is prepared by [user:ko_osaga,2020-01-01] [user:nong,2020-01-01] [user:ckw1140,2020-01-01]. I am personally very thrilled to deliver my first Codeforces contest as such a memorable one!
More credits for the contest:
* Coordinator: [user:300iq,2020-01-01]
* Testers: [user:kdh9949,2020-01-01] [user:kriii,2020-01-01] [user:OnionPringles,2020-01-01] [user:molamola.,2020-01-01] [user:evenharder,2020-01-01] [user:jihoon,2020-01-01] [user:DongwonShin,2020-01-01] [user:xia...
1. [user:mnbvmar,2020-01-04] 2. [user:TLE,2020-01-04] 3. [user:Benq,2020-01-04]
4. [user:tourist
Announcement of Hello 2020
100.
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...
You will have **3 hours** to solve **9 problems, one of which is divided into
two subtasks**.
Announcement of Codeforces Global Round 20
101.
[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...
= weight; i < dp.size(); i++) { 3 dp[i] -= dp[i - weight]; 4 } ~~~~~, calculations 2 for (int i = dp.size() - 1; i >= weight; i--) { 3 dp[i] += dp[i
- weight]; 4 } ~~~~~, #### 3. Square root optimization of knapsack/"$3k$ trick" Assume you have $n$
rocks with, . Currently $O(\log n)$. Called $O(n \sqrt n)$ times. 3. Find the minimum
element. Currently $O(\log n, 1. Insert an element in $O(1)$. 2. Delete an element in $O(1)$. 3. Find the
minimum element in $O, Given $n = 3 \cdot 10^5$ points on a plane, you get to pick a subset of 500
points and need to, Note that operations 1 and 2 are more frequent than 3, but they take the same
amount of time. To, ~~~~~ 1 for i in 1..n: 2 for (int k = i; k <= n; k += i): 3 # whatever ~~~~~
102.
Worst-Case Graphs for Maximum Flow Algorithms Maximum flow algorithms are known to perform significantly better in practice than their worst-case time complexities would suggest. So in this blog, I will present graphs on which the following maximum flow algorithms actually achieve their worst-case running time:
| Algorithm | Time Complexity |
| ----------------------- | ------------------------------ |
| Ford-Fulkerson with DFS | $\mathcal{O}\big(m^2 \cdot U)$ |
| Edmonds-Karp | $\mathcal{O}\big(n \cdot m^2\big)$ |
| Dinic | $\mathcal{O}\big(n^2 \cdot m\big)$ |
| DFS with Scaling | $\mathcal{O}\big(m^2 \cdot \log(U)\big)$ |
| Dinic with Scaling | $\mathcal{O}\big(n \cdot m \cdot \log(U)\big)$ |
| Most Improving Augmenting Paths | $\mathcal{O}\big(m \cdot \log(U) \cdot (m + n \cdot \log(n))\big)$ |
| FIFO Preflow Push | $\mathcal{O}\big(n^3\big)$ |
| Highest-Label Preflow Push | $\mathcal{O}\big(n^2 \cdot \sqrt{m}\big)$ |
Here and in the rest of this blog:
- $n =$ number of vertices,
...
\rightarrow h \rightarrow t$ | 5 | |3| $s \rightarrow e \rightarrow f
\rightarrow a \rightarrow b, And the graph sizes are not large enough, such that $\frac{1}{150} n^2 \cdot
\sqrt{m}$ beats $3, To achieve a running time of $\Omega(n^3)$ we will reuse this structure
$\Omega(n)$ times. To do, [3] Brian C. Dean, Michel X. Goemans, and Nicole Immorlica. "Finite Termination
of "Augmenting Path
103.
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...
1. [user:Benq,2020-10-10] 2. [user:yosupo,2020-10-10] 3.
[user:ksun48,2020-10-10] 4
Announcement of Codeforces Global Round 11
104.
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...
relevant synonyms of "problem". 3. Moreover, if you look the word up in more
traditional dictionaries
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 ...
anything! I plan to spend no more than 3 hours on answers. Sorry if I can't
answer all the questions.
106.
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...
3. [user:OguriCap,2021-09-18], 3. [user:peehs_moorhsum,2021-09-18]
Announcement of Codeforces Round 743 (Div. 1)
Announcement of Codeforces Round 743 (Div. 2)
107.
Catching silly mistakes with GCC As you know, the C++ language assumes that the programmer is always correct. Therefore C++ compilers don't add additional checks to the program, such as checks for null pointer dereference or out-of-bounds array access. This is good, because C++ programs run as fast as possible, and this is bad, because sometimes we may spend a long time debugging some silly mistake. We would want that the compiler can find such mistakes automatically. And many compilers can! In this post I will show various GCC options that do this. Previously [user:zakharvoit,2015-01-02] already wrote about this [here](/blog/entry/13875).
All options that will follow should be added to the GCC command line. In various IDEs you can do it in IDE or compiler settings. Many of the options can also be used with Clang (for example, in Xcode). For MSVC++, I think, there is nothing better than Debug mode and `/W4`.
[cut]
GCC warnings
------------------
Of course, the first step to debugging is to enable compi...
int main() { for (int i = 0; i <= 3; i++) { arr[i] = i; cout << arr[i, ~~~~~ int arr[3];, ~~~~~ int main() { int arr[] = { 3, 1, 2 }; cout << binary_search(arr, arr+3, 2, ~~~~~ int main() { vector v(3); cout << v[7] << endl; } ~~~~~
108.
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...
= {a[0] + b[0], ..., a[3] + b[3]} */ ```, b + \omega^2 c + \omega^3 d \\ a - \omega b + \omega^2 c - \omega^3 d \\ a +
\omega i b - \omega^2, vector $x[0]+y[0], \dots, x[3]+y[3]$, all in just one assembly instruction., $$ |z'-z| \leq |x| \cdot |y| \cdot 3\varepsilon k\left(2 + \sqrt
5\left(1+\frac{1}{3k}\right, $$\begin{gather*} r_{4k+1} &=& -r_{4k}, \\ r_{4k+2} &=& i r_{4k}, \\ r_{4k+3}
&=& -i r_{4k, **Experiment 3**: Put numbers to be uniformly picked from $[-15k, 15k]$. This
emulates balanced, , where we need to do 3 inverse transformations. With "2-in-1" approach this
will only reduce the number
109.
Codeforces Round 860 (Div. 2) Hello Codeforces!
I am happy to invite you to [contest:1798], which will be held on [contest_time:1798].
This round will be **rated for participants with rating lower than 2100**. Participants with a higher rating are invited to participate in the round unofficially.
You will be given **6 problems** and **120 minutes** to solve them. All problems were authored and prepared by me.
The traditional thanks-list to everyone who took part in the creation of the round:
🤴 [user:DishonoredRighteous,2023-03-19] for coordinating the round
🐞 [user:gyh20,2023-03-19] for black-red testing of the round
😈 [user:feecIe6418,2023-03-19], [user:iakovlev.zakhar,2023-03-19], [user:Dart-Xeyter,2023-03-19], [user:Adam_GS,2023-03-19], [user:felys,2023-03-19], [user:golikovnik,2023-03-19], [user:Gary2005,2023-03-19] for red testing of the round
🐫 [user:NemanjaSo2005,2023-03-19], [user:Alexdat2000,2023-03-19], [user:Kon567889,2023-03-19], [user:tem_shett,20...
3. [user:RGB_ICPC7,2023-03-26], 3. [user:neal,2023-03-26]
Announcement of Codeforces Round 860 (Div. 2)
110.
Codeforces Round #547 (Div. 3) Привет, Codeforces!
Я рад пригласить вас на [contest:1141], который начнётся во [contest_time:1141]. К официальному участию приглашаются все, чей текущий рейтинг строго меньше 1600. Все остальные — могут принять участие вне конкурса.
Так получилось, что расписание этого месяца не пестрит раундами (координаторы, мы надеемся на вас!), и поэтому я решил частично исправить ситуацию. Все задачи этого раунда придуманы и подготовлены мной в последний день сборов [Hello Muscat Programming Bootcamp 2019](/blog/entry/63298) и в перелетах из Маската в Санкт-Петербург. Я даже специально засёк время на подготовку: к текущему моменту (задачи готовы к тестированию) я потратил около 6 часов на их подготовку, включая придумывание некоторых из задач. Мне очень нравится работать над задачами, это что-то на стыке творчества и программирования. Очень надеюсь, что вам понравится результат моей работы.
<center>
<br/>
<img src="/predownloaded/76/ea/76eaf024dd0f6321a74e266ed77baad33aac32c0.png...
Codeforces Round #547 (Div. 3), **Штраф** за неверную попытку в этом раунде (и последующих Div. 3 раундах)
будет равняться **10
Announcement of Codeforces Round 547 (Div. 3)
111.
Codeforces Global Round 5 Editorial Enjoy!
<spoiler summary="A. Balanced Rating Changes">
[tutorial:1237A]
<spoiler summary="Solution by tourist">
~~~~~
#include <bits/stdc++.h>
using namespace std;
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
int n;
cin >> n;
int flag = 1;
for (int i = 0; i < n; i++) {
int x;
cin >> x;
if (x % 2 == 0) {
cout << x / 2 << '\n';
} else {
cout << (x + flag) / 2 << '\n';
flag *= -1;
}
}
return 0;
}
~~~~~
</spoiler>
</spoiler>
<spoiler summary="B. Balanced Tunnel">
[tutorial:1237B]
<spoiler summary="Solution by tourist">
~~~~~
#include <bits/stdc++.h>
using namespace std;
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
int n;
cin >> n;
vector<int> a(n), b(n);
for (int i = 0; i < n; i++) {
cin >> a[i];
--a[i];
}
for (int i = 0; i < n; i++) {
cin >> b[i];
--b[i];
}
vector<int> pos(n);
for (int i = 0; i < n; i++) {
pos[b[i...
(3 * n); for (int i = 0; i < n; i++) { cin >> a[i]; a[i + n] = a[i + 2 * n] =
a[i, ); for (auto &t : tt) if (t == 1 || t == 2) t = 3 - t; cnt[0] = cnt[1] = cnt[2, const int D = 3;
112.
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...
, 3, and 4 rounds regularly attract hundreds or even thousands of competitors
too high rated to
113.
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...
ended up in the contest, for the GR I think 3). The remaining problems were
created after my first, solve during the simulation. 3. Share with you a *detailed* feedback about the
problems., that one of the problems was known (AGC44 D). 3. I had fun interacting with
[user:antontrygubO_o,2020
114.
Blogewoosh #2 Hello, codeforces!
All signs in the sky and on the ground indicate that you've enjoyed my first blog, so here is the second one. I've decided to choose a Polish task again, as there are plenty of interesting ones. This time we'll take a look at the "plot purchase" ([you can submit here](https://szkopul.edu.pl/problemset/problem/hwbyoUkNFPXQLOPcw3x5huTR/site/?key=statement)), which is a bit easier, but a few years ago I was very proud of myself when I solved it. The statement goes as follows:
You are given a square $n \times n$ grid ($1 \leq n \leq 2000$). In every cell, there is a number from the range $[1, 2 \cdot 10^9]$. You are also given an integer $k$ ($1 \leq k \leq 10^9$). A task is to find a subrectangle of this grid such that the sum of values in this subrectangle lies in the range $[k, 2\cdot k]$ (or report that there is no such subrectangle). Just print coordinates of its opposite corners.
[cut]
$ $
If the problem would be more difficult, and we'd be asked to fi...
sum of values, then it'd be possible to find it in $O(n^3)$ time, just by
fixing lower and upper sides
115.
Codeforces: Итоги 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>
Привет, Codeforces!
Недавно платформе исполнилось <b>10 лет</b>. Мы принимаем ваши поздравления по ссылке <b><a href="https://codeforces.me/10years">https://codeforces.me/10years</a></b>. Пожалуйста, ознакомьтесь
с прогрессом в 2019-м году и <a href="https://codeforces.me/10years">поддержите краудфандинг-кампанию</a> по случаю юбилея. Ваше пожертвование станет вкладом в будущее Codeforces, поможет его развитию и работе. Нам нужны ресурсы,
чтобы продолжать проводить раунды, не останавливаться в разработке, внедрять нововведения и поддерживать существующую инфраструктуру.
В 2019-м году над задачами совместно с многочисленными авторами работали координаторы: [user:KAN,2020-02-06], [user:300iq,2020-02-06], [user:cdkrot,2020-02-06], [user:arsijo,20...
"/>
Индия впереди планеты всей! Россия откатилась на 3-е место, уступив 2-е место
Китаю
116.
Blogewoosh #3 Hello, codeforces!
This time I've decided to choose a task from my own contest which took place last April and was known as the Grand Prix of Poland. If you want to write this contest virtually in the future, then consider not reading this blog. If you've participated in this contest and maybe even solved this task, then anyway I recommend reading it, cause this task has many very different solutions, each of them being very interesting (in my opinion). It's also a reason why this blog is longer than previous ones.
I'll write about task C "cutting tree" (not uploaded to the ejudge yet :/). The statement goes as follows:
You are given a tree with $n$ vertices ($1 \leq n \leq 2 \cdot 10^5$). The task is to calculate $f(k)$ for each integer $k$ from the range $[1, n]$ where $f(k)$ is defined as the maximum number of connected components of size $k$ which we can "cut off" from the tree. A connected component of size $k$ is a set of $k$ vertices such that it's possible to traverse ...
Blogewoosh #3
117.
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] ...
3. [user:BurnedChicken,2024-03-05], 3. [user:INeedCarb,2024-03-05]
Announcement of Codeforces Round 932 (Div. 2)
118.
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...
) + 2`, `(3 << 16) + 3`, ..., into this hash table, all of the outputs will be
equivalent modulo $2^16
119.
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...
3. [user:ksun48,2026-05-23]
120.
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...
ago. It worked well in season 2, but season 3 has became more and more
competitive, we must
121.
Codeforces Round #812 (Div.2) Hi Codeforces!
==================
[user:_dlbm17,2022-06-25], [user:Demen100ns,2022-06-25], [user:Spyofgame,2022-06-25] and I are delighted to invite you to participate in [Codeforces Round #812 (Div. 2)](https://codeforces.me/contests/1713).
- Start time: [contest_time:1713]
- Duration: **120 minutes**.
- Number of tasks: **6**, including at least **one interactive** problem. Make sure to read [this blog](https://codeforces.me/blog/entry/45307) and familiarize yourself with these types of problem before the round!
<spoiler summary="This contest is brought to you by:">
- <a href = "https://www.facebook.com/HocvienPTIT">PTIT</a>, my university.
- <a href = "https://discord.gg/vKjE9xan">Code Mely</a>, a Vietnamese fanpage for Computer Science.
</spoiler>
<img align="right" src="https://i.ibb.co/VLCYnxH/received-1102672770650793-1.png" width="60%" height="60%">
Special thanks to:
- <span>[user:errorgorn,2022-08-02] for wonderful coordination</span><span style="col...
3. [user:5cd,2022-08-07], 3. [user:kotatsugame,2022-08-07]
Announcement of Codeforces Round 812 (Div. 2)
122.
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...
3. [user:Benq,2024-02-17]
Announcement of think-cell Round 1
123.
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 ...
1. [user:Petr,2017-12-29] 2. [user:dotorya,2017-12-29] 3.
[user:V--o_o--V,2017-12-29] 4
124.
Технокубок 2020 — Отборочный Раунд 1 (и открытые рейтинговые раунды Codeforces Round 591 Div.1, Div.2) Добрый день!
<p><a style="float:right;margin:0 0 1em 1em;" href="http://codeforces.me/technocup2020"><img src="http://assets.codeforces.com/images/technocup-narrow-logo.png"/></a>
В [contest_time:1223] состоится <a href="/contests/1223">Отборочный Раунд 1</a>
олимпиады для школьников <a href="http://codeforces.me/blog/entry/70187">Технокубок 2020</a>.
Раунд будет длиться два часа, участникам будут предложены 7 задач. По его результатам лучшие участники
(но не более 45% от общего числа участников раунда) будут приглашены на финальный этап в Москву.
Для регистрации на раунд и участия перейдите <a href="http://codeforces.me/contests/1223">по ссылке</a>.
Не забудьте заранее зарегистрироваться на раунд! Для опоздавших будет открыта дополнительная регистрация.
<p style="text-align:center">
<a style="font-size:20px;" href="http://codeforces.me/contests/1223">Зарегистрироваться на Отборочный Раунд 1 →</a>
<br/>
<small>Соревнование открыто для всех в виде отдельных ...
| Python 2, Python 3, PyPy2, PyPy3 | [submission:35883730] \(Py2),
[submission:36179112] \(Py3) | | Ruby
Announcement of Technocup 2020 - Elimination Round 1
125.
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";
}
}
...
Speed up finding the minimum cut using a segment tree. , Speed up the transition by storing the state in a segment tree, Using hint 3, can we break down the problem into smaller sub-problems, We start with only the root edge, then add edges of depth $0$, followed, Following the greedy approach for all 3 cases will result in a correct solution
that runs in $O(n
Tutorial of Hello 2024
126.
The 2026 Universal Cup Finals The best and brightest programmers from around the world have gathered this week in Shanghai, China, for the highly anticipated 2026 Universal Cup Finals!
<center>
<img src="/predownloaded/e6/93/e693e47e517f397b13fc0a1dab505067ca789e98.jpg">
</center>
### Event Details
- Date: Sunday, May 10th
- Start time: 11:00 AM CST (3:00 AM UTC)
- See [participating teams](https://codeforces.me/blog/entry/153064)
- Full schedule can be found [here](https://finals.ucup.ac/schedule/)
### Streaming
Witness the best contestants in the world as they push the boundaries of skill and strategy. You can be part of this event by following us online either on the [English livestream](https://www.youtube.com/watch?v=Y9HMX3zhFMM) or the [Chinese livestream](https://live.bilibili.com/1753191619).
<center style="margin: 2em;">
<iframe width="560" height="315" src="https://www.youtube.com/embed/Y9HMX3zhFMM" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipb...
- Date: Sunday, May 10th - Start time: 11:00 AM CST (3:00 AM UTC) - See
[participating teams
127.
Chinese Postman Problem: How I Ran a 180 km Ultramarathon Through Every Street in My City Hello, Codeforces!
A couple of years ago, a friend of mine told me that two cyclists had passed through every single street in the center region of the city I live in (Belo Horizonte, southeastern Brazil), on the [third try](https://www.strava.com/activities/1030665876/overview).
<spoiler summary="First try">
After ~160 km, they had not completed 1/3 of the goal. [Strava link](https://www.strava.com/activities/976021527).
<img src="/predownloaded/8a/53/8a53829f7d78e88808a3ff751b64406ec2fc0efd.png" style="width: 600.0px;max-width: 100.0%;max-height: 100.0%;">
</spoiler>
<spoiler summary="Second try">
They were better prepared, but one of the guys was run over by a car (no big injuries). [Strava link](https://www.strava.com/activities/999450285).
<img src="/predownloaded/85/8f/858fdcdcfda93cd1d9c7c59db7dbfad9e63d6ea2.png" style="width: 600.0px;max-width: 100.0%;max-height: 100.0%;">
</spoiler>
<center>
<img src="/predownloaded/8d/5c/8d5cfb677a361654105e49f36f214c35...
decomposed into the cycles $C_1 = (1, 3, 4, 2)$, $C_2 = (3, 5, 6)$, and $C_3 =
(4, 7, 8)$. After removing
128.
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.
...
1. [user:ainta,2019-07-20] 2. [user:Radewoosh,2019-07-20] 3.
[user:tzuyu_chou,2019-07-20] 4
Announcement of Codeforces Global Round 4
129.
GPT Can Code, but Can It Panic at Pretest 3? <h3>The Setup</h3>
You: “Solve this problem.”
GPT: “writes 200 lines of perfectly formatted <code>C++17</code> with descriptive variable names and a poetic docstring.”
You: “It’s WA on test 3.”
GPT: “Have you tried using long long?”
---
<h3>How GPT Thinks It Codes:</h3>
> "I analyzed 10^8 lines of ICPC gold-medal solutions and learned optimal dynamic programming patterns."
<h3>How GPT Actually Codes:</h3>
> "Let's just sort the array and pray."
---
<h3>Debugging with GPT</h3>
You: “It’s TLE.”
GPT: “Hmm, maybe use <code>unordered_map</code>.”
You: “Still TLE.”
GPT: “Then switch to <code>map</code> for more structure.”
You: “It’s slower.”
GPT: “At least it’s elegant.”
---
<h3>GPT’s Logic in Action</h3>
Test 1: Works.
Test 2: Works.
Test 3: Segmentation fault.
GPT: “That’s impossible. Theoretically flawless.”
Then it proudly explains:
> "Maybe the problem statement is wrong."
> — confidently, like a true C...
GPT Can Code, but Can It Panic at Pretest 3?, GPT doesn’t panic at Pretest 3. GPT doesn’t cry over -17 rating. GPT doesn’t
scream “IT
130.
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...
number of needed rounds (recently it is pinely 3, goodbye, hello and I predict
that there might not be
131.
The Reason You are Bad at Codeforces — You are Not Russian Enough If you are triggered by this clickbaity blog title, you are probably interested in improving your Codeforces skills. Now, I will share my point of view on the differences between Codeforces and ICPC contests, and how you can, in my humble opinion, maximize your Codeforces rating gain.
I mostly consider that all conceivable competitive programming problems need three aspects of skill:
1. Observation — the ability to understand the problem and come up with non-trivial properties.
2. Technique — the ability to apply a well-known algorithm or data structure to the problem.
3. Implementation — the ability to code fast and debug fast.
Collaterally, I refer to these three skills as Russian-ness, Chinese-ness and American-ness, respectively, for reasons you will soon see below.
## Observation (Russian-ness)
Observation means that you stare at some problem for a sufficient amount of time and you are able to reduce it to some easier problem. One good proble...
3. Implementation — the ability to code fast and debug fast.
132.
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...
### Part 3. The time complexity
133.
Nebius Welcome Round (Div. 1 + Div. 2, rated, t-shirts!) Hello, Codeforces!
[user:Zlobober,2023-03-09] and I are glad to invite you to compete in Nebius Welcome Round (Div. 1 + Div. 2) that will start on [contest_time:1804]. The round will be rated for everyone and will feature 8 problems that you will have 2 hours to solve.
I feel really thrilled and excited about this round as this is the first time I put so much effort in a Codeforces contest since I quit being a round coordinator in December 2016 (wow, that was so long ago)!
We conduct this round to have some fun and we also hope to find some great candidates to join Nebius team. Solving 5+ problems in our round will be a good result and will count as one of the coding interviews.
Apart from that top 25 contestants and 25 random contestants placed 26-200 will receive a branded Nebius t-shirt!
<img style="float: right; margin: 0 1em 0.5em 1em; width: 400px;" src="/predownloaded/40/37/403770e78d9c8962d0bc0c20a9e1c45d0e3c5df9.png">
Some information about Nebius. I have join...
3-rd place: [user:Um_nik,2023-03-12]
Announcement of Nebius Welcome Round (Div. 1 + Div. 2)
134.
The Time Complexity Lower Bound for Segment Tree Beats is log^2 Original Post: https://jiry-2.blog.uoj.ac/blog/1404
The following content is translated by Gemini 2.5 Pro.
## Part 1: Introduction to the Hacking Method
### How to Map the Hack Sequence to a Segment Tree
First, we are given a Hack sequence of length $len$. After $O(1)$ range addition operations and $\Theta(\log len)$ global $\text{chkmax}$ operations, this sequence becomes a cyclic shift (rotation) of the original sequence. Since a cyclic shift does not affect our range addition and global $\text{chkmax}$ operations (we can simply shift the indices for the range addition accordingly), we can consider the sequence as unchanged.
If we have such a sequence, let $len=n^{\frac{1}{3}}$, and place such a sequence at intervals of $n^{\frac{2}{3}}$. Each sequence corresponds exactly to a segment tree interval. A subtree of size $n^{\frac{2}{3}}$ contains exactly one such segment tree interval.
For the Hack sequence, the range addition operations are performed one by one on eac...
## Part 3: Miscellaneous Talk, If we have such a sequence, let $len=n^{\frac{1}{3}}$, and place such a
sequence at intervals of $n, The total number of range addition and global $\text{chkmax}$ operations is
$O(n^{\frac{1}{3}}+\log
135.
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...
bit of math, you'll find that the answer on a perfectly random shuffle should
be $\frac{N}{3
136.
Codeforces Round #649 (Div.2) Hi everyone!
Codeforces round #649 will take place on [contest_time:1364]. It's rated for the second division, but, as usual, first division participants can take part out of competition.
The problems were created by me. I'd like to thank my forever orzable coordinator [user:antontrygubO_o,2020-06-11]; my incredible army of testers [user:dorijanlendvaj,2020-06-11], [user:300iq,2020-06-11], [user:Osama_Alkhodairy,2020-06-11], [user:AmShZ,2020-06-11], [user:Taran_1407,2020-06-11], [user:TigranMec,2020-06-11], [user:_Aaryan_,2020-06-11], [user:Mohammad_Yasser,2020-06-11], [user:zoooma13,2020-06-11], [user:lavish315,2020-06-11], [user:Utkarsh.25dec,2020-06-11], [user:NOOBxCODER,2020-06-11], and [user:Laggy,2020-06-11]; and, of course, you-know-who for the amazing codeforces and polygon platforms.
This time, in an effort to kill type-races and because I'm lazy, you'll be given 5 problems and 2 hours to solve them.
**UPD:** the scoring distribution will be 750-1000-1500-2000-2500...
1. [user:Um_nik,2020-06-13] 2. [user:kefaa2,2020-06-13] 3.
[user:hank55663,2020-06-13] 4
Announcement of Codeforces Round 649 (Div. 2)
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