I Love CF, I miss the old CF, Anton rounds cold CF Textbook sized blogs CF, no cheater
blogsCF I hate, I miss the sweet CF, debug till sleep CF I gotta say pre-GPT I’d like to meet CF
See we, I used to love CF, I used to love CF I even made the rounds, I thought I was CF
What ifCF, That’s all it was CF, we still love CF And I love you like CF loves CF
→ 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.
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
2.
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_...
; using namespace std; using namespace std::chrono_literals; using namespace cf
; int main, using namespace std; using namespace std::chrono_literals; using namespace cf;
Announcement of Codeforces Round 1119 (Div. 3)
3.
I don't want this to get normalized I don't think comments like "you use X in model solution, author, are you an idiot?", "f**k you and your problems, they are shit" and such should be normal on Codeforces, especially when written by someone high-rated and/or respected in the community. I also don't think this kind of behavior should be normal towards regular users.
I know that sometimes contests can be frustrating. Sometimes it is the participant's fault, sometimes it is the problemsetter's fault, sometimes it just happens without anyone being guilty of that. And I understand that in some cases, the criticism the author receives is fair and well-deserved.
But there is a fine line between criticism, saying that you didn't like the problems, and hurling insults against the author. The former two are acceptable (and sometimes even needed, because authors have to improve and learn from their mistakes); the latter one, in my opinion, should not be acceptable.
It's not like we can ever get rid of offense and hatred ...
hatred on CF, so it's much easier to find the examples of that. But this
behavior towards regular, that the problemsetters usually get the most of the hatred on CF, so it's much
easier to find the
4.
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...
objective, the statements will be about 8 people who triumphed the most times
inCF rounds. Using the, . As I wanted to be objective, the statements will be about 8 people who
triumphed the most times inCF
Announcement of Hello 2019
5.
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-...
Happy New Year (both on CF and irl... maybe go touch some grass for the first
time this year)!, nonnegative number of problems on the round. We hope you have a Happy New Year
(both onCF and irl... maybe go
Announcement of Good Bye 2025
6.
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...
ones too (for instance, CF 1069, CF 1058)., 1. I work as a coordinator on CF, but this blog is solely my initiative (I
didn't tell anyone from, CF (and usually samples are very strong), but I don't think it's a bad thing to
have from time to, Hi, a few disclaimers first. 1. I work as a coordinator on CF, but this blog is
solely my
7.
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...
of his rounds is far below the average of CF Rounds. This great coordinator
has coordinated the, rounds is far below the average of CF Rounds. This great coordinator has
coordinated the following
8.
IOI vs CF performance: 2024 Edition <blockquote>
CF ratings are unrelated to your IOI results. CF contests differ significantly from IOI contests.
</blockquote>
❌ **Wrong!**
A higher CF rating, with a gap of at least 100, correlates with an **87%** chance of achieving a better placement at IOI. In fact, the relationship between IOI placement and CF rating is strongly connected. The [Pierson Correlation Coefficient](https://en.wikipedia.org/wiki/Pearson_correlation_coefficient) for these two factors is -0.83.
<center>
<be>
<a href="https://algoritmiacademy.com/wp-content/uploads/2024/10/output_at_ioi.png">
<img width="80%" src="https://algoritmiacademy.com/wp-content/uploads/2024/10/output_at_ioi.png">
</a>
<br>
<i>Correlation between CF rating and IOI placement</i>
</center>
❗ What is that outlier participant on the top-right? He is Jiyu Shen ([user:PubabaOnO,2024-10-02]), with CF rating of 3000+, but it seems that he was unable to attend the contest 💔
Here are the statistics...
IOI vs CF performance: 2024 Edition, ">
Correlation between CF rating and IOI placement , ** in CF rating!, -10-02]), with CF rating of 3000+, but it seems that he was unable to attend
the contest 💔, /output_before_a_year.png">
Correlation between CF rating a year before IOI and IOI placement, A higher CF rating, with a gap of at least 100, correlates with an **87%**
chance of achieving a, CF ratings are unrelated to your IOI results. CF contests differ significantly
from, Here are the statistics on CF ratings for each medal:
9.
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...
Nature of Modern Cults on CF — A
Documentation](https://codeforces.me/blog/entry/90871) - [If, # Have Some Fun - [The Mysterious Nature of Modern Cults on CF — A
Documentation](https
10.
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...
creating CF., least, [user:MikeMirzayanov,2015-12-28] provides two awesome platforms — CF
and Polygon. And, platforms — CF and Polygon. And there are so many people involved in
Codeforces. Thank you
Announcement of Good Bye 2015
11.
Support Anton Trygub Since [user:antontrygubO_o,2020-07-12] has became CF coordinator he is trying hard to make rounds which meet his standards for quality and (more importantly) beauty. I think he is doing a great job.
For some time there is a very vocal group of people who dislike his view on things. I feel like it will be hard for Anton to maintain his work if he would think that majority of people doesn't approve.
But I don't really think that this is a majority. If you enjoy rounds prepared or coordinated by Anton, please comment here with some positive feedback.
`#AdHocProblemsMatter`
Since [user:antontrygubO_o,2020-07-12] has became CF coordinator he is trying
hard to make rounds
12.
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]...
-10], [user:MrDecomposition,2020-04-10] - **CF Admins**: Thanks to
[user:MikeMirzayanov,2020-04-10, ], [user:Maripium,2020-04-10], [user:MrDecomposition,2020-04-10] - **CF
Admins**: Thanks to
Announcement of Codeforces Round 633 (Div. 1)
Announcement of Codeforces Round 633 (Div. 2)
13.
Who sorry_dreamoon really is Hello everybody. So I was really bored tonight and looked at so many comments being like "[user:sorry_dreamoon,2015-02-18] is X" and then people arguing and disagreeing. Since I had nothing better to do, I decided to finally unravel the truth of who he actually is. Here is the full case and how I actually got to him, hope you enjoy! :D
Selection
------------------
Let's start by the obvious. He managed to get first place in Div2 and he got second in Div1 (would've been first if the scores were static). So what we know is that he is really good.
Let's look at the past 10 Div1 contests. It turns out that only 8 times it happened that someone with a rating of less than 2500 got in top5. This is 8 times out of 50 people being in top5. But he was second and almost first, so he is better than that.
It turns out actually, that in the past 10 Div1 contests, only 2 times it happened that someone with rating less than 2500 got in top3.
Combining this information with [user:sorr...
out a few times, the obvious way is to remove all people that participated in
CF rounds #284 and #292, As pointed out a few times, the obvious way is to remove all people that
participated inCF rounds
14.
Unimportant Um_nik Update Hello Codeforces! I have some big news.
I am trying to fight depression and live a fuller life. That includes some self-realization, maybe even (I don't even want to think about it) a job. I love cp, and I love teaching, and I know I already tried that, but I believe that this is the way.
With the help of my wife, I started to work on some related projects, and today we want to share them with the world. All of them are in the early stages, but we set a hard deadline for March 1, so...
Um
==================
Um... I am starting a series of educational YouTube videos / selected problems.
Every episode of Um is focused on one problem. I will explain how to solve this problem and talk about some related concepts. The idea is to show off some cool concepts using real problems.
The kicker: the problem is made public at least a week before the episode airs. So you can use Um as a source of problems. I will try to choose nice problems that highlight some ideas, and problems ...
don't want to break CF rules more than I already did, so... If you are
interested, read the [tutoring, I don't want to break CF rules more than I already did, so... If you are
interested, read the
15.
Codeforces Round #300 (с призами!) Привет, Codeforces!
[В воскресенье, 26-го апреля в 19:00 MSK](http://www.timeanddate.com/worldclock/fixedtime.html?day=26&month=4&year=2015&hour=19&min=0&sec=0&p1=166) состоится 300-ый регулярный раунд Codeforces.
Я поздравляю всех членов и администрацию Codeforces с этой замечательной цифрой. С момента создания платформа стала гораздо больше и круче, провела множество соревнований и предоставила возможность любому интересующемуся человеку прокачаться в знании алгоритмов и решении сложных задач. За это мы благодарим создателя платформы Codeforces [user:MikeMirzayanov,2015-04-24] и всю команду Codeforces. Жгите дальше!
Я рад объявить, что задачи трехсотого юбилейного раунда составил я, Михаил Тихомиров ([user:Endagorion,2015-04-24]). Возможно, вы помните прошлые раунды на моих задачах: [#99](http://codeforces.me/contests/138,139), [#109](http://codeforces.me/contests/154,155), [#265](http://codeforces.me/contests/464,465), [#283](http://codeforces.me/contests/497,496) и (частич...
103 144 152 161 169 198 233 243 269`. The lucky participants are as follows:
[user:bmerry,2015-04, 103 144 152 161 169 198 233 243 269`. Вот счастливчики, занявшие эти места:
[user:bmerry,2015-04-26, 13 14 15 40 46 67 75 80 102 103 144 152 161 169 198 233 243 269`. The lucky
participants are as, 269`. Вот счастливчики, занявшие эти места: [user:bmerry,2015-04-26],
[user:Egor,2015-04-26
Announcement of Codeforces Round 300
16.
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...
#### 6. Here is C++0x in CF, why still C++?
17.
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...
the second part is completely trivial, then the problem shouldn't be used in
officialCF contest, trivial, then the problem shouldn't be used in official CF contest (maybe in
Educational Round only
18.
CF Sweatshirt signed by legends at ICPC World Finals Good morning! ICPC World Finals just concluded in Moscow. I was lucky enough to be participate representing the University of Central Florida, who finished in a respectable 17th place.
While there, friends, acquaintances, and CF Legends signed my Codeforces sweatshirt in their handle with their current color. The most common request of the people I asked was to show off what it ended up looking like on a blog post of Codeforces, so here that is!
<spoiler summary="Famous Signings">
</spoiler>
#### Signers:
- [user:MikeMirzayanov,2021-10-11] the creator and maintainer of Codeforces and Polygon who deserves our eternal thanks, and [user:Una_Shem,2021-10-11] without whose work there would be no sweatshirt to sign.
- [user:tourist,2021-1...
CF Sweatshirt signed by legends at ICPC World Finals, understandable mistake since they are the same color on CF. , , friends, acquaintances, and CF Legends signed my Codeforces sweatshirt in
their handle with their, -11]** - [user:brunomont,2021-10-11] who I've discussed some CF blogs with and
his teammate, While there, friends, acquaintances, and CF Legends signed my Codeforces
sweatshirt in their handle
19.
Good Bye 2022, Hello 2023! <img src="/predownloaded/cf/40/cf40c72e68b2aee5005a91f39e1d3404d917617b.jpg" style="float:right; margin:0 1em 1em 1em;"/>
Hello, Codeforces!
The year 2022 is finally coming to an end. Let's be clear. It's been a terrible year. Tomorrow I will make a wish, and you can easily guess which one.
I am writing these lines, and in a few minutes the Good Bye 2022 round will begin. And this inspires optimism and even pride in the community. Despite all these difficulties and troubles, we make rounds together and participate in them. Let's be grateful. And for this, let's say symbolic thanks to 2022 and a lot assistants: coordinators, problem writers, testers, participants and sponsors. We are many and we are together.
Can I have a little more detail on the Codeforces team?
Let me separately say thanks and words of admiration for the work of the coordinator of coordinators [user:KAN,2022-12-30] and the senior developer [user:geranazavr555,2022-12-30]. Both of you bring a lot to Cod...
20.
Introducing CF Stonks — A Virtual Stock Market Based on Your CodeForces Rating! **2/11/2023 Update**: There had been a month-long hiatus due to the CF name change magic (and my own laziness oops), but everything has been fixed now :D! However, I know a lot of people have gone inactive due to the month-long market closure, so I am hoping to **relaunch StonksCF** in the near future with some new mechanisms (dividends, automatic algorithm trading, etc.). Let me know if you have any ideas! In the meantime, feel free to continue exploiting the way-over-inflated economy and embrace your inner capitalism >:D
**Final Update**: We now have our own **DISCORD SERVER**. Join us for the latest information and engage with the community at [https://discord.gg/AeH2GTwkF5](https://discord.gg/AeH2GTwkF5) All the announcements will be there instead of on this page
Greetings Fellow CodeForces users!
I am proud to introduce my latest project, [CF Stonks](https://codetiger.me/project/StonksCF) (aka StonksCF), which is a virtual stock market based on your CodeForces rating.
...
Introducing CF Stonks — A Virtual Stock Market Based on Your CodeForces Rating!, **2/11/2023 Update**: There had been a month-long hiatus due to the CF name
change magic (and my, I am proud to introduce my latest project, [CF
Stonks](https://codetiger.me/project/StonksCF) (aka
21.
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...
and around 20 in regular Open Cup seasons. Unlike 2-3 hour CF rounds, these
are 5-hour contests for
22.
Sums and Expected Value — part 1 part 2: https://codeforces.me/blog/entry/62792
Watch my lecture-stream tomorrow (Thursday) at [14:00 CEST](https://www.timeanddate.com/worldclock/fixedtime.html?msg=EV+lecture+1&iso=20181025T14&p1=262) — <s>[https://www.youtube.com/watch?v=qdlPY37MBPo](https://www.youtube.com/watch?v=qdlPY37MBPo)</s> [https://www.youtube.com/watch?v=U_h3IjreRek](https://www.youtube.com/watch?v=U_h3IjreRek).
I will go through theory and problems from this blog.
The only prerequisite is knowing what is probability.
The next (harder) part on Monday.
The video will be available later, with timestamps for each problem — so you don't have to watch everything.
### Definition of EV
Let's say we bought a lottery ticket for 2$.
We will win 10$ with probability 10%, and 20$ with p-bility 2%.
On average, it gives us $0.1 \cdot 10 + 0.02 \cdot 20 = 1.4$,
so we are worse off after buying the ticket.
The computed average is called the expected value.
The expected value (EV, expecta...
+7$. 5. **Imbalanced array (CF Educational Round 23)** Same as the previous
problem, but choose a, 5. **Imbalanced array (CF Educational Round 23)** Same as the previous problem,
but choose a, 6. **Randomizer (CF 313 D)** You're given a convex polygon with $N$ vertices
($N \le 2000$ or $N
23.
On AI ruining "solving math problems with computer" [a year later] I guess you were right in the part "a year from now LLMs will solve problems on your level". You were also saying something along the lines of "you'll change your mind when it starts affecting you". In this part, you were wrong.
Cheaters always existed, and they always will. LLMs make it easier to cheat, true. There is no way of completely preventing LLM cheating (or any other kind of cheating) in online competitions.
I just coordinated the round everyone is buzzing about, so I can with confidence say that in the CF rules for coordinators and authors, there isn't anything about making problems LLM-proof. Not for easy problems, not for hard problems, nothing. And I doubt any such rules will appear. LLMs are forbidden by the rules, that's that.
So we, as honest participants (yes, I will assume that you are an honest participant, otherwise this blog is not applicable to you and I don't care about you), will have to accept that some problems in the future rounds will be LLM-able, ...
confidence say that in the CF rules for coordinators and authors, there isn't
anything about making, open standings during the contest, if you can get a solve count for every
problem. Heck, in aCF round, I just coordinated the round everyone is buzzing about, so I can with
confidence say that in theCF
24.
(Request) Update PyPy's Version on CF I've always liked using Python (PyPy) for solving problems in competitive programming. And most problems are very doable, even in Python. What I've found is that the most difficult problems to solve in Python are those requiring 64 bit integers.
The reason why 64 bit integers are problematic is because CF runs Windows, and PyPy only supports 32 bit on Windows. So whenever a problem involves integers that cannot fit inside of a signed 32 bit int, PyPy switches to big integers (which runs insanely slow, sometimes a factor of 20 times slower).
<spoiler summary="What I currently have to do to get around big integers">
### 1. Switching to floats
What I usually try is switching from using integers to floats. This works if all integers are $\leq 2^{52}$:
[submission:53441394] (Fat TLE and almost MLE) vs [submission:53456822] (AC by a mile)
### 2. Calculating `a * b % MOD`
Many problems on CF involve doing multiplication modulo $10^9 + 7$. This is super slow because of big integ...
(Request) Update PyPy's Version on CF, because CF runs Windows, and PyPy only supports 32 bit on Windows. So whenever
a problem involves, usable and more beginner friendly. So if possible please update PyPy's version
onCF to 7.3.4, ### 2. Calculating `a * b % MOD` Many problems on CF involve doing
multiplication modulo $10^9 + 7, The reason why 64 bit integers are problematic is because CF runs Windows, and
PyPy only supports
25.
Hacking (as a CF feature) does not fit modern competitive programming 1. How does hacking work with the new feature of unrated registration? I have seen this question raised in the comments to the announcement several times, but there is no answer still and I think it's weird that Mike hasn't thought about it before announcing.
2. I was working recently on a div.2 Codeforces round (still might happen in the future) and I was told by my coordinator that pretests should be equal to systests in all the problems. From this I can conclude that it is a Codeforces policy and intentionally weak pretests do not happen. It doesn't mean that it is impossible to hack anybody, but it doesn't sound like a viable strategy. I find it weird that this information is not public, it's like I'm getting bonus information about all other Codeforces rounds by setting a round myself.
I have done exactly zero research on the matter, but my feeling is that 99% of hacks (I'm talking about during-the-round-hacking here, not open hacking of education rounds and such) happen in ...
Hacking (as a CF feature) does not fit modern competitive programming
26.
Codeforces Round #269 (Div. 2) Добрый день Codeforces! Рад пригласить всех к участию в Codeforces Round #269 для второго дивизиона, который состоится [в эту пятницу, 26-го сентября](http://www.timeanddate.com/worldclock/fixedtime.html?day=26&month=9&year=2014&hour=19&min=30&sec=0&p1=166). Как обычно программисты первого дивизиона приглашаются поучаствовать неофициально (читай "за респект"), от своего имени очень прошу первый дивизион участвовать.
Задачи будут все мои, так что вы знаете, кого ругать, если задачи вам не понравятся. Со своей стороны я постарался приложить максимум усилий, чтобы избежать подобного.
Это мой первый раунд, так что я представлюсь. Мне 27 лет, уже лет 7 работаю программистом, но вещами типа ACM/ICPC не занимался и открыл их для себя относительно недавно. Пару лет назад узнал словосочетание "dynamic programming" с курса на Coursera и чуть позже открыл для себя Codeforces, с тех пор я пропадаю тут. Я родился и вырос в Санкт-Петербурге, прожил там до 24-х лет. А потом женился, и мы пе...
Codeforces Round #269 (Div. 2), Hello Codeforces community! I'm happy to invite you to participate in
Codeforces Round #269 for, Википедия просила напомнить вам, что день, когда состоится этот раунд, будет 269
-м днем года (по, Добрый день Codeforces! Рад пригласить всех к участию в Codeforces Round #269
для второго дивизиона
Announcement of Codeforces Round 269 (Div. 2)
27.
On problemsetting 2 [Previous part](https://codeforces.me/blog/entry/70178)
Hi there again! Previous part with opinions of some amazing problemsetters turned out to be interesting for you, so I decided to gather some more opinions :D. I really hope that this post will help someone in their problemsetting future!
#### When and how did you start problemsetting?
[user:300iq,2020-01-11]: April, 2017
[user:maroonrk,2020-01-11]: In 2015 Summer, I hold the first contest with friends at high school club. It was a local Japanese contest.
[user:Radewoosh,2020-01-11]: In high school, so probably for 5 years. I was creating problems for other people from my school as my teacher told me that it's a completely different point of view.
[user:voidmax,2020-01-11]: It was 3 years ago. I was on plane and I didn't have a clue what to do in my free time. Then I realized that I can try myself in problemsetting. Later I created first and one of my favorite problem ([problem:963B]). I was full of inspiration a...
#### What is your opinion about subtasks? Is this format suitable in CF
/TC/AtCoder contests??, So I don't think Subtasks are suitable for CF/TC since they have drain points
system. And even if
28.
Codeforces Round #741. How contest author is entertaining. Hello, Codeforces! Not too long ago there was my second round, Codeforces #741 (Div. 2). Before it started, I wondered: surely there will be a lot of cheating on my round, just like on any other? So I decided to do something unusual.
As you know from [a blog about cheaters] (https://codeforces.me/blog/entry/93893), it's easy to find telegram/youtube channels with cheaters. So I decided to find a couple and join them...
So, the first telegram group I found was [link not available yet]. There weren't too many cheaters in it but they were, here's an example:
<spoiler summary="Picture">
<a href="https://imgbb.com/"><img src="https://i.ibb.co/kJqMzMS/pic1.png" alt="pic1" border="0"></a>
</spoiler>
I joined this telegram group, and there was a meaningful dialogue:
<spoiler summary="Picture">
<a href="https://ibb.co/1mC2csG"><img src="https://i.ibb.co/VqFB6HY/pic2.png" alt="pic2" border="0"></a>
</spoiler>
This may seem unbelievable, but the admin of the group actuall...
fight cheating on CF: to close such telegram/youtube channels. Otherwise there
is simply no way to
29.
[RANT] Warning to other coordinators about Wuhudsm Hi again, it is me. I want to preface this blog by saying that I am totally aware that it is not good netiquette to screenshot private messages and publish them publicly. But honestly, I can't care anymore now.
Below is an unorganized rant. Sorry, I'm too pissed to write the things below concisely. And here: I'm pinging him: [user:wuhudsm,2024-07-21].
In an ideal world, I could probably just tell him privately to stop doing it. But I already did. So I think I need to write this blog to either stop him from doing this again or just soft-ban him from authoring more rounds by telling everyone about it.
Let me just preface this blog with a fact of coordination -- it is very possible that problems from contest A will be used in contest B even after testers from contest A have seen those problems. So there are indeed (many) testers who could possibly be affected by this. Some examples of this happening (where I was involved):
- Contest problems based on on-site contests were quit...
CF are both well-respected for having enjoyable problems, but a good problem for
CF may not be a
30.
CF Round #333 <center><img src="https://lh3.googleusercontent.com/--HN6DjhjZaM/VlNt6NQ6olI/AAAAAAAAAuQ/w6wSlj7rl0k/s250-Ic42/"></img><div>(<a href="http://i2.kym-cdn.com/photos/images/newsfeed/000/582/289/38f.gif">оригинальная версия</a>)</div></center>
Привет всем!
CF раунд #333 (обе дивизионы) состоится [сегодня](https://www.timeanddate.com/worldclock/fixedtime.html?day=24&month=11&year=2015&hour=19&min=35&sec=0&p1=166). Авторы раунда — я и [user:Baklazan,2015-11-18].
Просто случайно, мои задачи пронумерованы чётно и задачи Баклажана пронумерованы нечётно. И тепер можете раздумать, если они пронумерованы от 0 или от 1 :D.
Как обычно, благодарим [user:GlebsHP,2015-11-19] за его помощь (в частности, за помощи упростить некоторые задачи), [user:MikeMirzayanov,2015-11-19] за CF и Polygon, [user:Delinur,2015-11-24] за перевод условий задач на русский язык и тестерам [user:misof,2015-11-24], [user:Mimino,2015-11-24], [user:AlexFetisov,2015-11-24] и [user:winger,2015-11-24].
Желаю хо...
CF Round #333, the problems), [user:MikeMirzayanov,2015-11-19] for CF and Polygon,
[user:Delinur,2015-11-24] for, некоторые задачи), [user:MikeMirzayanov,2015-11-19] за CF и Polygon,
[user:Delinur,2015-11-24] за, ">оригинальная версия) Привет всем! CF раунд #333 (обе дивизионы) состоится, CF round #333 (both divisions) is going to take place
[today](https://www.timeanddate.com, CF раунд #333 (обе дивизионы) состоится
[сегодня](https://www.timeanddate.com/worldclock
31.
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-...
to CF rating
32.
[Tutorial] Square Root Techniques _This is my 100th CF blog!_
This is a list of techniques with $O(\sqrt n)$ time complexity.
Watch the lecture https://youtu.be/BJhzd_VG61k, with timestamps!
1. Square root decomposition — split the sequence into blocks of fixed size.
2. Splitting objects (e.g. vertices) into light and heavy.
3. Square root decomposition by the time of queries & rebuilding the structure.
4. Mo's algorithm — processing queries in proper order and updating the answer by erasing/inserting new elements. https://cp-algorithms.com/data_structures/sqrt_decomposition.html
5. Strings — if the sum of lengths is $S$ then there are at most $\sqrt{S}$ distinct lengths.
6. Birthday paradox & baby-step giant-step. See P4 and P6 [here](https://codeforces.me/blog/entry/95571), and see https://cp-algorithms.com/algebra/discrete-log.html.
P1. [problem:398D]
P2. [problem:220B]
P3. <s>[problem:86D]</s> (actually, skip this one because it's boring)
P4. Count triangles in a graph, i....
_This is my 100th CF blog!_
33.
<h1>Mike, please stop ignoring your userbase</h1> **This blog will compile a list of codeforces issues along with all the times Mike has ignored complaints about them**
## 1. Unreasonable delay for viewing submissions [<a class="rated-user user-green">+973</a>]
- [Submission pages on Codeforces often not loading](https://codeforces.me/blog/entry/134655) [<a class="rated-user user-green">+281</a>] by [user:jeroenodb,2025-04-04] on *October 2024*
- [Unreasonable delay for viewing submissions](https://codeforces.me/blog/entry/135457) [<a class="rated-user user-green" title="Green 392">+392</a>] by [user:djm03178,2025-04-04] on *October 2024*
- [Please, give us proper explanations about this nonsense.](https://codeforces.me/blog/entry/136830) [<a class="rated-user user-green">+300</a>] by [user:djm03178,2025-04-04] on *December 2024*
No [user:MikeMirzayanov,2025-04-04] reply
## 2. Cloudflare misconfiguration [<a class="rated-user user-green">+1232</a>]
Posted [CF should rename itself to Cloudflare instead of Codef...
- [CF not working on our wifi](https://codeforces.me/blog/entry/140863) [, ="rated-user user-green">+1232] Posted [CF should rename itself to Cloudflare
instead of, It is not unreasonable to ask for accountability when CF has become the
de-facto competitive, Posted [CF should rename itself to Cloudflare instead of
Codeforces](https://web.archive.org/web
34.
TL Issue in the #659 (Div. 1) Problem F I've hacked all AC solutions of [this](https://codeforces.me/contest/1383/problem/F) problem, which were submitted during the contest, and I believe I can hack most of upsolving solutions.
Currently, I only checked that they time out on my local machine, because "Unexpected Verdict" prevents me from uphacking.
I even hacked model implementation in the editorial.
[Here](https://ideone.com/cHvjkp)'s my latest generator. Feel free to challenge it.
Side Note: [user:yosupo,2020-07-26] wrote a [solution](https://codeforces.me/contest/1383/submission/88036534), which passes the above case, but I failed it with another [generator](https://ideone.com/LKx3FP).
What I want to argue is that TL of this problem is too tight, and it affected some of the competitors.
For example, I got almost the same idea as the model solution during the contest. Still, I was too scared to write it because I suspected it would time out without full optimization and possibly some tweaks like shuffling ve...
by how fast your solutions run on your test.** I wish future CF writes and
coordinators read this, .** I wish future CF writes and coordinators read this, and that's the reason
why I posted it as a
35.
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...
). This didn’t happen here. This was addressed after being pointed out by
participants in [CF
36.
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...
are quite new for competitors in CF although they are well known in China from
the final standings
37.
Four Proposals Regarding FSTs While I very much enjoy Codeforces rounds overall, FSTs are by far my least favorite part of the Codeforces contest format. There are two reasons I dislike contests with weak pretests. The first is stylistic: I don't care very much about penalizing people who don't rigorously prove their solutions before submitting (which is the main justification for having weak pretests) and I think it's essentially impossible to FST these solutions without more frequently FSTing contestants who make minor mistakes in their code. (The vast majority of my FSTs are due to this sort of minor glitch rather than an error in the underlying idea).
The second is that FSTs introduce an element of hidden information to Codeforces rounds. During a contest, you have no way of knowing whether the authors have written strong pretests. If you act as though the pretests are weak, you will be at a disadvantage if pretests are actually strong (because if you knew pretests are strong, you could have e.g. submit...
this applies to the strong majority of CF setters.)
38.
Apology for faults in Codeforces Round 1098 Hello, I'm Sanae.
In ufc+8 zone, I watched the entire contest at midnight. Too many cheaters emerged and we were busy banning them. I noticed WA submissions on [problem:2228B], [problem:2228C2] and TLE submissions on [problem:2228D], which was intended. Soon after the end of the contest, I released the editorial. The bad news was, the complaints flooded the website and my contribution fell from +91 to -92. After reflecting on the issues, I was anixous and blamed myself. Now, I make an apology to point out the problems to avoid them next time. Sorry, everyone!
1. Too many cheaters emerged.
[Comment 1](https://codeforces.me/blog/entry/153617?#comment-1365371)
2. The problems stay in the queue for a very long time bothered contestants.
3. B is easy but many contestants misssed the corner case. And C has many corner cases.
[Comment 1](https://codeforces.me/blog/entry/153617?#comment-1365212),
[Comment 2](https://codeforces.me/blog/entry/153617?#comment-1365256),
[Comment 3...
it in samples at CF. But I didn't even realize C has many corner cases. I set
$a$ up to $10^{17, Upd on 2026.05.22: I left CF for a whole week and when I return, I find so many
new comments have
39.
Most annoying person on CF It is this time of year! Many people trying to summarize all the things happened during the year. And since I love to hate people, I want to make a vote for most annoying person on CF in 2018.
I know that I'm in top contributors list for too long, so to vote for given person you should **downvote** the comment with this person's handle(s). Due to the nature of voting through comments on CF, you can vote for many people, and you can also upvote comments if you find given person's comments helpful/funny/positive.
If you care for your contribution but want to nominate a candidate, you can PM me their handle(s), then I will write a comment to vote. Note that I **will** write that it was you who nominated this person, so it saves your contribution but not relationships with the nominee. I will ignore messages from people without rating.
If you hate someone already nominated so much that you want them to know, you can also PM me so I'll add you to list of people who nominated given ...
Most annoying person on CF, **downvote** the comment with this person's handle(s). Due to the nature of
voting through comments onCF, . And since I love to hate people, I want to make a vote for most annoying
person onCF in 2018.
40.
Catching the Mole II(Hard Version) (Psychotic_D cheating case aftermath) This is a part 2 of [my other blog](https://codeforces.me/blog/entry/131821) on the [user:Psychotic_D,2024-07-23] situation. I highly recommend you see it before this one for context.
If you don't care that much, here is a quick recap of what was covered in the other blog before we get started: [user:Psychotic_D,2024-07-23] had a ridiculously good performance on his friend's ([user:wuhudsm,2024-07-23]) round. This let many people become suspicious of whether he achieved this legitimately. Then multiple accusations were thrown around and I did my blog doing my own investigation and compiling the most compelling evidence I found. The takeaway was that [user:Psychotic_D,2024-07-23] was probably cheating along [user:EndlessDreams,2024-07-23], who would give him code for [user:Psychotic_D,2024-07-23] to copy. [user:Psychotic_D,2024-07-23] would alter them heavily to avoid plagiarism, but it was clear beyond a reasonable doubt that he was cheating.
Well, I finished writing that blog, ...
/) website. However, I only entered the cf group for they had on it. This time,
I entered their, It was brought up that on a random cf round, another master [previously accused
of cheating](https, cf account to discord if you dont have access to it. He is certainly the one
that had access to the
41.
Huawei IOI Camp 2022 Hi. I'm organizing an online IOI-preparation camp in collaboration with Huawei.
Every day will be a 5-hour virtual contest, then my problem analysis, and sometimes a lecture.
There will be at least 4 contests and 2 lectures.
We will do some old IOI contests (2013?) from the [IOI archive](https://codeforces.me/blog/entry/104593), CEOI (2015-2016?), or maybe JOI/JOISC — to be decided this weekend.
I don't want to do recent years like 2021 because most participants already covered it.
The example lecture topics are those from Range Queries or Trees in [Usaco Guide Platinum](https://usaco.guide/plat).
The camp is free for IOI 2022 participants.
For everybody else, the video recordings will be posted on Youtube after the camp.
By participating you get the live experience (Discord, asking questions, competing with others, leaderboards).
**Dates**: 25.07-2.08 (updated)
**Time** of every analysis/lecture: [14:00 UTC / 16:00 CEST](https://www.timeanddate.com/worldclock...
to [user:SlavicG,2022-07-11] via CF or Discord (SlavicG#6969). UPDATE: please
use discord only, ?"> If you're an IOI 2022 participant or team-leader, reach out to
[user:SlavicG,2022-07-11]via CF, For everybody else, write to me via CF or email (find it
[here](https://www.youtube.com/c/Errichto
42.
Slope trick explained _Special thanks to [user:Ari,2020-05-13] for helping me proof-reading this blog :D_
Hi everyone! Some years ago I read [user:zscoder,2020-05-12]'s tutorial on slope trick. Initially, I found the blog a bit difficult to understand, so I came up with a different explanation and system for my personal use. Today, I finally have the courage to share it with you guys. So without further ado, let's get started!
#### Formulation and properties
Slope trick is a way to represent a function. Here, I denote that a function is **slope-trick-able** (sorry for the lack of creativity) if the function satisfies 3 conditions:
1. It is continuous.
2. It can be divided into multiple sections, where each section is a linear function with an integer slope.
3. It is a convex/concave function. In other words, the slope of each section is non-decreasing or non-increasing when scanning the function from left to right.
For example, the function $y = f(x) = |x|$ is a slope-trick-able function ...
43.
CF Hot News: Seattle Builds galen_colin orz Statue CF Hot News: Seattle Builds galen_colin orz Statue
==================
Good morning everyone!
We have just received word that the City of Seattle has completed their recently funded [user:galen_colin,2020-11-25] orz statue. City planners say that this is the latest update in their recent endeavor to honor their favorite CF personalities.
"I'm proud to announce that the galen_colin orz statue has finally reached completion," announced city planner Vasya. "You know, after we decided to shape the space needle as a symbol praising the great [user:Monogon,2020-11-25], some of us thought that we would never be able to make such an amazing work of art again. We're truly delighted that we were able to prove all the skeptics wrong with this new piece."
The project was built entirely using renewable energy created by water turbines run exclusively on the tears of java programmers TLEing on system tests. The city hopes that,...
CF Hot News: Seattle Builds galen_colin orz Statue, endeavor to honor their favorite CF personalities.
44.
[Tutorial] Generating Functions in Competitive Programming (Part 2) Welcome to Part 2 of my tutorial on generating functions. The [first part](https://codeforces.me/blog/entry/77468) focused on introducing generating functions to those without any background in generating functions. In this post, I will demonstrate a few applications of generating functions in CP problems. Let us start with some relatively straightforward examples.
Note: Unless stated otherwise, all computations are done modulo a convenient prime (usually $998244353$). Also, $[n]$ denotes the set $\\{1,2,...,n\\}$.
### Blatant Applications in Counting Problems
**Problem.** [AGC 005 Problem F](https://atcoder.jp/contests/agc005/tasks/agc005_f) You have a tree $T$ with $n$ vertices. For a subset $S$ of vertices, let $f(S)$ denote the minimum number of vertices in a subtree of $T$ which contains all vertices in $S$. For all $1 \le k \le n$, find the sum of $f(S)$ over all subsets $S$ with $|S| = k$.
Constraints: $n \le 2 \cdot 10^{5}$.
<spoiler summary="Solution">
First, ...
This problem was created $6$ years ago and has a 3100 rating on CF
45.
The 2025 Universal Cup Finals — Wrap up We are excited to share that The 2025 Universal Cup Finals just successfully wrapped up in Dongguan, China!
<center>
<img src="/predownloaded/cf/81/cf81069734f8d9b6e994f27c84e2781ccbe20c38.jpg">
</center>
What a wonderful week! In our five-day event, participants went to the city tour of Guangzhou, competed in the Challenge session, participated in the Conference for Competitive Programmers, and most importantly, our main competition — The 2nd Universal Cup Finals!
<div class="row">
<img src="/predownloaded/f5/d8/f5d8cd3b6264dfe7de13433662a7a1fbe61293f0.jpg" style="height: 262.0px;max-width: 100.0%;max-height: 100.0%;">
<img src="/predownloaded/cb/40/cb4020f1b2f88fc7cd7e40f11ccda90c88df26ba.jpg" style="height: 262.0px;max-width: 100.0%;max-height: 100.0%;">
</div>
<div class="row">
<img src="/predownloaded/88/82/88820a183af6e038ba219c5887674276cd29322d.jpg" style="height: 262.0px;max-width: 100.0%;max-height: 100.0%;">
<img src="/predownloaded/8d/ab/8daba...
46.
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...
" online round. I was in doubt between an AGC and a CF Global Round. - 20-21
April: I decided to go
47.
Pinely Round 1 (Div. 1 + Div. 2) <img align="right" alt="text" src="/predownloaded/1c/91/1c91ed031100f3338aa5f8ed58d0405adb9ff9d7.png" style="height: 500.0px;max-width: 100.0%;max-height: 100.0%;">
Hello, Codeforces!
Members of team EZEC are glad to invite you to participate in **[contest:1761]**, which will start on **[contest_time:1761]**. You will be given **7** problems, **one of which has a subtask**, and **2 hours and 30 minutes** to solve them. The round will be rated for everyone. It is greatly recommended to read all the problems.
There is at least one interactive problem, so please see the [guide of interactive problems](https://codeforces.me/blog/entry/45307) if you are unfamiliar with it.
We would also like to thank:
- Our red sun [user:orzdevinwang,2022-11-19], the youngest TOP10 coder in history, for illuminating our path.
- Our great emperor [user:antontrygubO_o,2022-11-19] for cooperating and coordinating with us to make this round happen. He saved the round by contributing his problem...
registered [here](https://codeforces.me/ratings/organization/31353) on CF.
Announcement of Pinely Round 1 (Div. 1 + Div. 2)
48.
The 3rd Universal Cup. Stage 38: Guangzhou (CCPC Finals 2024) Hi everyone! We are thrilled to invite you to The 3rd Universal Cup, Stage 38: Guangzhou, taking place on May 24th, 2025.
The contest is based on the Finals of the CCPC (China Collegiate Programming Contest), which was held on May 11th, 2025. This is my third time to be a judge on the CCPC Finals, and probably my last experience of my judging experience in high school.
This contest is prepared by [user:Gellyfish,2025-05-22], [user:johnvictor,2025-05-22], [user:jqdai0815,2025-05-22], [user:Kevin114514,2025-05-22], [user:Kubic,2025-05-22], [user:lynkcat,2025-05-22], [user:Milmon,2025-05-22], [user:ODT,2025-05-22], [user:orzdevinwang,2025-05-22], [user:Qingyu,2025-05-22]， [user:rddccd].
Although the style of problems differs a lot, the difficulty of this problem set is close to the ICPC World Finals or Regional Semifinals (NERC Finals or Asia EC Finals). We believe the problems will be engaging for participants of all levels while still providing a challenge for the strongest te...
] ([CF](https://codeforces.me/team/172147) and
[UCup](https://qoj.ac/user/profile/ucup-team001)).
49.
One-letter variables Complete shitpost, don't take it seriously. We were discussing `#define int long long` and I suggested defining it to one letter if you want to save on typing so desperately. [user:jeroenodb,2025-10-11] said that those are reserved for variables, which prompted me to think, "Is there a letter I don't standardly use for some particular purpose?" Then I compiled the following list, and then I thought, "Well, I already wrote it, let's put it on cf", so here we are.
~~~~~
a - array, but also all sorts of stuff
b - second array
c - third array / count
d - delta / divisor
e - event / edge
f - flag
g - graph / second flag
h - third iterator / third flag / height
i - iterator
j - second iterator
k - third input size / some parameter (i know, those are all parameters, but for some reason k is parameter)
l - left
m - number of edges / second input size
n - input size
o - ???
p - permutation / position
q - number of queries / query / second permutation / second position
r - r...
thought, "Well, I already wrote it, let's put it on cf", so here we are.
50.
Click here if you want to know your future CF rating Have you ever wanted to know how will your CF rating look like in the future, without having to be stressed about the potential shortcomings which may come in future contests?
Have you ever wanted to be able to say that you will get the rating of your dreams soon?
If the answer for both of these questions is Yes, then you came to the right place.
During the last few months, I have used some technical analysis to predict how will some of the people from [my Discord server](http://discord.gg/z3PcA8aDmg) and I have gotten quite consistent results, so I think that my strat is working.
I am not going to enter in details on how do I actually do this, because it requires quite some experience to learn all the details, but as people say, an image is worth 1000 words, so here are some few examples.
First example is [user:ScarletS,2020-12-29]
Here is how his rating looked like in November, before he had a bad contest
Here I told him t...
Click here if you want to know your future CF rating, Have you ever wanted to know how will your CF rating look like in the future,
without having to be, If you want to know how will your(or your friends') CF rating will look like,
post the handle in
51.
First time becoming red In yesterday's contest [contest:1815] I've got the achievement which I could not imagine just half a year ago — reach 2400 rating and become GM.
The first time I learned about CF was on 2022.2, that time I registered this account and took my first contest. But my first submission [submission:146390589] on contest got TLE, and I solved 0 problem in this contest. (At that time I didn't know that's because the efficiency of `java.util.Scanner` is extremely low) And because I needed to deal with graduation thesis, I didn't use CF until the end of last year, when I restarted CF contest.
At 4th and 5th contests I solved div2D problems to reach blue. In my 9th contest, I first time solved div2E problem in contest, and I became purple in my 12th contest. In my 14th and 15th contest, I solved div2E problems twice and became orange. In the next weeks, I started to hit the upper limit of my skill, and dropped from orange twice.
In order to improve my skill of competive programming...
a year ago — reach 2400 rating and become GM. The first time I learned about CF
was on, In order to improve my skill of competive programming, I practiced problems in
CF problemset, The first time I learned about CF was on 2022.2, that time I registered this
account and took my
52.
Streaming my push for LGM Hi! It's been a while. My YT has certainly changed direction, but that's beside the point. I have been "silently" grinding for LGM over the past few months and sporadically earlier — avoiding explicitly doing contests, but doing plenty of virtuals and general practice. For now, I would put myself in the 2800s rating-wise (for a more detailed and dynamically updated log of the virtuals, see [here](https://docs.google.com/spreadsheets/d/1ocT7fbWmAoJCdRV05mVyJ6RBLo8LF9xAFkALdYfIIw8/edit?usp=sharing#gid=827979571/)).
I want to hold myself accountable better. Getting myself to wake up is pretty damn hard. So I've opted to stream my grind over my winter break from college (roughly 3 more weeks of break, and probably streaming throughout school too), not only for myself, but as some sort of motivation for everyone to work harder (perhaps to create an environment where one can tune in and grind for themselves alongside).
Logistics: I won't have a set start time, it will be soon aft...
Plan: I will probably grind problems from recent CF rounds for a while, then
finish each stream
53.
On setting D2A and D2B problems Disclaimer: as always, everything in this blog is just my opinion.
Some problemsetters don't take easy problems seriously. One very popular opinion is that the D2A-B problems are just stupid implementation exercises intended for beginners to learn how to code.
It's true that D2A-B problems are mostly like this on most platforms. This is one of the reasons why a lot of people don't like combined rounds: they think that solving D2A-B problems can't be interesting and just takes time which could be spent on solving fascinating problems of higher level. From point of view of most participants, easy problems just can't be good, because if problem requires some thinking than it's not easy enough for D2A-B level. Easy problems can't be interesting, they say!
This situation really upsets me. This kind of attitude from the ...
proposal on CF :P From my side as a coordinator, I try to not let boring
problems get intoCF rounds
54.
How to ask for help in PM There was a blog today with a similar title, and although it was (I think, not really sure as I didn't get a good look) some kind of satire/joke blog, I wanted to post some serious advice about things I (don't) like to see when people are PMing me. However I decided that this wouldn't be in the spirit of the blog and decided to make a separate blog. And now that blog is deleted anyway.
While it's not a bad thing to write messages to more experienced users, most people who write me do things that make them very difficult to deal with. These are my opinions, but somewhy I feel that many reds will agree. Some things here feel silly to write because they are so obvious. But I'm only writing them because people regularly mess them up.
[cut]
$~$
####Use punctuation and spelling.
There are some [very simple rules](https://codeforces.me/blog/entry/82186#comment-689836) that make your messages infinitely more readable. I find it especially strange when many of these messagers call...
information is important. There are countless examples of CF blogs where some
green asked for help
55.
I Reached GM after 5 years Hello Codeforces! I did it! My main goal for 2022 was to reach red and I succeeded in the last contest of the year by getting +1 delta and going from cursed 2399 to 2400! I was very happy but scared to celebrate because I was worried that my rating would go down to 2399 again after the cheater removal. But now it’s 2401, so I think I’m safe now.
Why am I writing this blog? I'm very excited about this achievement, it means a lot to me and I want to share my happiness with Codeforces. Also people ask me in DM quite often about how I got onto this level or who I am in general, so I think it's time to summarize it all into one blog post.
Oh, wait… You don’t even know me, so why should you keep reading this blog post? Let me introduce myself a little first. I’m a high schooler from a small European country where few people do Codeforces. I love competing in OI and MO, I always did. One day, my father found the Codeforces website and suggested that I should start practicing there. At ...
encourage me to do CF. But I didn’t give up, and it paid off! Now I have more
motivation than ever to, it on the max test case on CF custom invocation. Tip: Use Clang++20 to check
for overflows/out-of, try to pick myself challenging problems (in the range around CF 2500-2800, but
that’s not very
56.
Похоже, против LLM-читинга мы смотрим не на то ### 1. Что именно за проблема
Во время раундов появилась конкретная схема: участник получает у LLM готовый код и просто отправляет его, почти не понимая.
Это не всегда ловится плагиатом: человек может изменить переменные, поменять местами некоторые части кода.
### 2. Идея
#### Xeppelin — open-source tool, изначально сделанный для ICPC, который:
- запускается локально перед контестом
- логирует изменения файлов в рабочей директории
- сохраняет таймлайн разработки (когда и как менялся код)
- не записывает экран, клавиатуру, сеть или вкладки
- после контеста строит визуализацию активности
#### [Это не мой tool, но вы можете найти его тут](https://github.com/KiK0S/xeppelin)
### 3. Что именно предлагается для Codeforces (реализация, не идея)
Предложение — использовать Xeppelin как внешний источник поведенческих данных, без вмешательства в CF-инфраструктуру.
Поток работы:
1. Участник запускает Xeppelin перед раундом
2. Решает задачи как обычно (локально, в ...
можно напрямую связать с CF-раундами и использовать как дополнительный сигнал
против копипаста LLM, — использовать Xeppelin как внешний источник поведенческих данных, без
вмешательства вCF-инфраструктуру. Поток, ### 7. Why this is safe for CF - no proctoring - no interference with
submissions - no mandatory, ### 7. Почему это безопасно для CF - нет прокторинга - нет вмешательства в
сабмиты - нет, #### Compromise solution (without changing the core CF system), #### Xeppelin works completely outside, CF receives data after the contest or
only upon request., #### Xeppelin работает полностью снаружи, CF получает данные после контеста или
вообще только по, #### Компромиссное решение (без изменения базовой системы CF), - не меняет процесс сабмита - не получает данные в реальном времени - не
требует изменений вCF и, . It can be directly linked to CF rounds and used as an additional signal
against LLM-solution copy, CF at the same time:, CF infrastructure. Workflow:, CF при этом:, CF-инфраструктуру. Поток работы:, This log can be automatically linked with CF submissions (by contestId + handle), Этот лог можно автоматически связать с CF-сабмитами (по contestId + handle)
57.
Where the author of the contest came up entertaining. Cheating cheaters. Hello, Codeforces! A month ago I have wrote [my blog about cheaters](https://codeforces.me/blog/entry/94300). Under that blog, a number of suggestions have accumulated, and there is a number of interesting ideas that could be applied to sabotage a group of cheaters.
I've often seen comments along the lines of:
<spoiler summary="Pictures">.
<a href="https://imgbb.com/"><img src="https://i.ibb.co/r65zfTL/cheat1.png" alt="pic1" border="0"></a>
<a href="https://imgbb.com/"><img src="https://i.ibb.co/Yh6ZfY8/cheat2.png" alt="pic2" border="0"></a>
</spoiler>
Well, doing what [user:Ari,2021-09-28] suggested wasn't really what I wanted to do, but it was worth checking out what would happen if the wrong solutions were sent. I asked some famous user what would happen if I sent the wrong solutions to cheaters. He told me that someone had done this before and that he was quickly debunked.
I wondered whether I could cheat cheaters without getting caught. But for this, I had to do so...
My first idea was to create a group on CF and invite all cheaters there. Then I
decided, that it's
58.
IOI vs CF performance: 2025 Edition <blockquote>
Popular opinion: CF ratings are unrelated to your IOI results. CF contests differ significantly from IOI contests.
</blockquote>
❌ **False!**
<blockquote>
Unpopular opinion: A contestant's Codeforces rating is more predictive of their final IOI result than their score on Day 1 is of their score on Day 2.
</blockquote>
✅ **True!**
A higher CF rating, with a gap of at least 50, correlates with an **84%** chance of achieving a better placement at IOI. The relationship between IOI placement and CF rating is strongly connected. The [Pierson Correlation Coefficient](https://en.wikipedia.org/wiki/Pearson_correlation_coefficient) for these two factors is **-0.813**. At the same time, the Pierson Correlation Coefficient between the scores of day 1 and day 2 of IOI is **0.804**.
A side note for IOI participants: If you performed poorly on the first day, just think of it as a single bad Codeforces contest—you can always do better next time! You can lik...
IOI vs CF performance: 2025 Edition, Popular opinion: CF ratings are unrelated to your IOI results. CF contests
differ, challenge. The **0.804** coefficient shows a strong connection, but just as CF
rating!, >
Correlation between CF rating and IOI placement , > Correlation between CF rating a year before IOI and IOI placement , A higher CF rating, with a gap of at least 50, correlates with an **84%**
chance of achieving a, Here are the statistics on CF ratings for each medal:
59.
IndiaHacks Finals — rated CF round Hi everybody.
The IndiaHacks finals will take place tomorrow (on Saturday).
The contest is being organized by HackerEarth.
Just after the official finals, the CF round will start ([check-your-time](http://timeanddate.com/worldclock/fixedtime.html?day=19&month=3&year=2016&hour=11&min=35&sec=0&p1=166)) with almost the same problems.
<s>You can treat it as a standard CF round — there will be 5 problems in each of 2 divisions, and 2 hours to solve them.</s>
Two divisions will compete together on the problem set with 7 problems, with 2 hours to solve them.
I want to especially thank [user:I_love_Tanya_Romanova,2016-03-18] for testing the problems, [user:GlebsHP,2016-03-18] for help with making the CF round possible, and [user:MikeMirzayanov,2016-03-18] for his awesome attitude and for the Polygon system.
Setters: [user:lewin,2016-03-18], [user:k790alex,2016-03-18], [user:Sokolov,2016-03-18], [user:Errichto,2016-03-18].
Testers: [user:I_love_Tanya_Romanova,2016-03-18], [use...
IndiaHacks Finals — rated CF round, HackerEarth. Just after the official finals, the CF round will start
([check-your-time](http, being organized by HackerEarth. Just after the official finals, the CF round
will start ([check-your, , [user:GlebsHP,2016-03-18] for help with making the CF round possible, and
[user:MikeMirzayanov,2016-03-18, Some info only for 40 official finalists — Remember not to discuss anything
until theCF
Announcement of IndiaHacks 2016 - Online Edition (Div. 1 + Div. 2)
60.
Click here if you want to know your future CF rating [Part 2] Have you ever wanted to know how will your CF rating look like in the future, without having to be stressed about the potential shortcomings which may come in future contests?
Have you ever wanted to be able to say that you will get the rating of your dreams soon?
If the answer for both of these questions is Yes, then you once again came to the right place.
Over the last three years since I posted [the first such blog](http://codeforces.me/blog/entry/86074) , everyone wanted to learn how will their CF rating look like in the future and I got such questions pretty much everywhere I could possibly get, including but not limited to:
- replies to comments I made here
- DMs here, on Discord and even on other social media platforms
- replies to messages on Discord
- even when I met people during onsite contests
As a result, exactly three years after the first iteration of the blog, I decided to come back with a second edition of this blog, where I am using my now three m...
Click here if you want to know your future CF rating [Part 2], /86074) , everyone wanted to learn how will their CF rating look like in the
future and I got such, Have you ever wanted to know how will your CF rating look like in the future,
without having to be, If you want to know how will your(or your friends') CF rating will look like,
post the handle in
61.
The 2023 ICPC World Finals Luxor <img src="/predownloaded/60/aa/60aa49ecdb6a668539163c4d7d797ac6f1bb43b3.jpg" align="right" style="height: 350px; margin: 10px 20px 20px;" alt="text"/>
Hello Codeforces!
The 2023 ICPC World Finals Luxor will begin on [April 18, 2024 at 10:00 UTC](https://www.timeanddate.com/worldclock/fixedtime.html?msg=2023+ICPC+World+Finals+Luxor&iso=20240418T12&p1=4626&ah=5). This time, we have a double World Finals. You can distinguish them by colors: blue (fish emoji) and green (crocodile). Join us on the live broadcast for this awesome double event of the year in competitive programming; commentators will include [user:ecnerwala,2024-04-17], [user:SecondThread,2024-04-17], [user:Egor,2024-04-17], and more!
ICPC World Finals Luxor is hosted by The Arab Academy for Science, Technology, & Maritime Transport (AASTMT), the ICPC World Finalist teams for two seasons will compete for World Championship awards, prizes, and bragging rights. More than 130 teams of each season represent the best of gr...
list with CF rating: [46](https://codeforces.me/blog/entry/116946)
[47](https://codeforces.me/blog
62.
Rumination on AI and competitive programming An OpenAI model can now achieve a rating of 1800 in Codeforces. That made me wonder about the future of competitive programming and why we do this. I came up with some opinions, which I'll be glad to share with you.
## AI killed the competitive programming?
I don't think that's happening, because competitive programming is useless! Some people might think CP is less interesting because AI can solve CP problems — but come on, problemsetter can solve those problems, unless they are really bad. Many people still play Chess just fine, and I recently enjoyed participating in Sudoku competitions, although I was completely aware that computers would wreck me in a second. It didn't matter, and I didn't see how it could.
CP problems are not "open problems" — if they are, the problemsetters are to blame. So, you are not contributing anything to the world by solving CP problems; it all comes down to personal interest. As long as CP is fun for people, they will do it.
## Flag...
allowed in CF, and notably, [I took advantage of
that](https://codeforces.me/blog/entry/93018, ban AI in online contests. Then, a CF rating will "really" be a number,
similar to the online judge, to cheat in various online contests with AI. That is not good news for CF and
AtCoder, which, years. No need to mention thousands of low-rated cheaters in CF. High-rated
people also cheat ([1, * CF skill might be very independent of one's skill for IOI / ICPC. * For most
users (and probably
63.
CF-Predictor — Знай свой рейтинг! <spoiler summary="UPD 02.09.2018">
Сегодня выпустил небольшое обновление в связи с увеличившим количеством пользователей, если вдруг заметите проблемы в работе, пишите, пожалуйста, сюда в комментарии или мне в личку, или в телеграм.
</spoiler>
Всем привет! Я на прошлом контесте предлагал всем желающий потестировать мой сервис предсказания изменений рейтинга. Сейчас я рад представить его!
Огромное количество Ваших нервных клеток погибает, так и не дождавшись обновления рейтинга. Хватит это терпеть! Теперь Вы можете использовать данный сервис для приблизительно вычисления изменения рейтинга.
Наиболее интересная составляющая — расширение для хрома. Оно изменять страницу положения, добавляя предсказываемые дельты. Расширения доступны для 3х браузеров:
[](https://chrome.google.com/webstor...
CF-Predictor — Know your rating changes!, CF-Predictor — Знай свой рейтинг!, I've deployed an update today. If you face any issues with CF, [Project](https://github.com/WslF/CF-rating-prediction) consist, [Проект](https://github.com/WslF/CF-rating-prediction, | CF-PredictorBackEnd | Java8, Tomcat | | 2 | web role | CF-PredictorFrontEnd
| Java8, Tomcat | | 3, изменений рейтинга. Сейчас я рад представить его! , **AWESOME UPD** Предсказание для сегоднешнего контеста (cf #399) полностью
соответствует реальным, ://addons.opera.com/en/extensions/details/cf-predictor/) [.  [here](http://cf, Prediction for todays contest (cf #399) is absolutely matching real rating
changes! Thanks for, Thanking [user:KieranHorgan,2017-10-20] now CF-Predictor has a new design.
Check [it](https, Расширение в работе: ![ ](https://github.com/WslF/CF
-rating-prediction/blob/master/Files
64.
NBHEXT — Know Your Rating Few weeks ago there was yet another discussion about "omg why we wait for the rating updates so long!?!?!" in Russian version of CF. [user:MikeMirzayanov,2015-10-30] said that there are several reasons:
1. cheaters are checked manually;
2. rounds are usually held in the evening and Mike spends some time to get home from the University;
3. sometimes his little daughter asks him to play with her and Mike can't refuse of course
After [user:meh,2015-10-30], [user:MaximM,2015-10-30] and me read the third reason, we started to think what can be done about it. After a little thinking we came up with a solution that seemed to make contestants and Mike's daughter happy. The solution was a creation of browser extension that maintains rating changes of a current scoreboard whenever you refresh the result page. Something like this:
Luckily, CodeForces provided us with all the tools to make it happen: an API a...
bugs you reported, add requested features, switch to backend in order to
decrease pressure onCF, long!?!?!" in Russian version of CF. [user:MikeMirzayanov,2015-10-30] said
that there are several, Несколько недель назад на просторах блогов CF было очередное обсуждение на тему
"почему так долго
65.
Problem Likeability Feature Proposal
--------
The feature to "star" or "favorite" a problem already exists; if you go to the problemset archive, you can choose to mark any problem as favorite. Same holds for marking any contest.
Unfortunately, it seems the only use for this feature, is so one could revisit his favorite problems in the future — you can't view the favorite problems of other participants. I think this feature can be easily extended to something more meaningful.
Maintain for each problem, the number of accounts who liked it (now referred to as the likeability of the problem). Then implement the option to filter (or sort) problems by their likeability (hopefully filtering problems by likeability could be combined with sorting by the number of people solved, so that people could find their own difficulty level). Same could go for contest likeability.
Pros
--------
- I think this can mainly help beginners. I hope that people who try out competitive programming for the first time, ...
- I believe this will improve the CF problemset as a source of practice, which
is probably a main
66.
MIT Mystery Hunt Disclaimer: This blog is not about competitive programming; you probably will not learn anything about algorithms and data structures. In other words, it doesn't qualify for [this initiative](https://codeforces.me/blog/entry/110840), but I want to give a link to it anyways because I think it's cool.
I love solving problems. No, seriously. I LOVE it. The mindset described in [this blog](https://codeforces.me/blog/entry/91114) is 100% how I feel. But it is not constrained to competitive programming problems. I loved math problems for a much longer time, and physics was somewhere there. But I also love solving puzzles. And playing puzzle games on the computer (well, on consoles). And participating in intellectual games. I am not as good in them as in cp, but it was never about being good for me, I just love the process.
And I think that there might be some people with a similar love for puzzles among competitive programmers, this is why I decided to write this blog on CF <s>and no...
programmers, this is why I decided to write this blog on CF and not because
this is the only place on
67.
Round 449 statements (warning: butthurt) If you don't wanna see some butthurt, skip this post.
1 December: I open main page of codeforces to see a picture of some anime girl taking half of my 1920x1080 screen. I got disgusted back than, but hatred for anime is my personal opinion, but IMO this picture was way too big to be on the main page of CF(it was the biggest one I saw on CF, if i remember correctly).
2 December: after nice Trial Round of NEERC I was looking forward to a CF round. I was not in my hometown and had internet which was LTE, but still was kind of laggy and slow. I was checking in in the hotel at the time of start of the round.
So the round starts, problem A loads, and here we go. There is another f***g anime picture right in front of me (which loads slowly because of internet, and the statement isn't loading until the picture loads (it generally isn't that way though)). Name -> Picture -> Statement. At this point i instantly added some curse words in my template (luckily i decided not to compete, so...
page of CF(it was the biggest one I saw on CF, if i remember correctly). 2
December: after nice, picture was way too big to be on the main page of CF(it was the biggest one I
saw onCF, if i, pictures(I don't know for sure because it's not very polite to talk to
coordinator ofCF about CF during, 2 December: after nice Trial Round of NEERC I was looking forward to a CF
round. I was not in my
68.
Pinely Round 4 (Div. 1 + Div. 2) <img src="/predownloaded/25/f9/25f953f1e0d453d352b1a0bde08014923fce0577.png" align="right" style="height: 200px;" alt="header"/>
Hello, Codeforces!
We are pleased to invite you to [contest:1991], which will start on [contest_time:1991]. The round will be rated for **everyone**.
You will be given **9 problems** and **3 hours** to solve them. There is at least one interactive problem, so please read the [guide for interactive problems](https://codeforces.me/blog/entry/45307) if you are unfamiliar with it.
The problems were authored and prepared by me.
We would like to thank:
- [user:errorgorn,2024-7-24] for the excellent coordination.
- [user:dXqwq,2024-7-24], [user:StarSilk,2024-7-24], [user:jiangbowen_,2024-7-24], [user:Endagorion,2024-7-24], [user:JCY_,2024-7-24], [user:jiangbowen,2024-7-24], [user:QwertyPi,2024-7-24], [user:chen_zexing,2024-7-24], [user:YocyCraft,2024-7-24], [user:zeliboba,2024-7-24], [user:HackerMonk,2024-7-24], [user:axs7384,2024-7-24], [user:Vlad...
Learn more about us on our [website](http://pinely.com) or find [our employees
onCF](https
Announcement of Pinely Round 4 (Div. 1 + Div. 2)
69.
[Feature request] Some suggestions for blogs I wanted to suggest some improvements for the blog system. Some of these things have probably been said before. Some of these things should be pretty easy to implement, others are harder and might require a complete rework. I tried to put the easy things first.
[cut]
$~$
#### Let's do something about necroposting
This is probably my biggest pet peeve about Codeforces blogs. Most of the time, Recent Actions is filled with very old blogs, some of which have been brought up for pretty much no reason at all. Currently, we have [useful C++ "library](https://codeforces.me/blog/entry/8387) from 2013 in Recent Actions, where the recent comment answers an old question which has already been answered better. Similarly, there is [How to add friends?](https://codeforces.me/blog/entry/2006) which was brought up to make some joke that has already been done.
The thing is though, a lot of the time these blogs are not brought up by trolls or anything like that. They are brought up by hel...
- if a blog is against the rules, it will be moved to drafts by CF moderation
(this is already
70.
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...
Let's say it does happen in 5 years, what happens to CP then? Assuming that
sites likeCF don't go, The only motivation for this tendency is to ensure that CF ratings and
leaderboards in contests
71.
Please improve QoL for blogs I'm pretty sure I've spent more time writing blogs than contests on Codeforces. Over time, I've found plenty of rough edges. So this is a plea to the Codeforces team to fix them.
I'm effectively going to say "please redo everything" over dozens of paragraphs, but I'll hopefully make the steps to the solution tangible and clear. If this is deemed way too ambitious, few improvements are better than none.
[cut]
<!-- -->
## Publication time
So I write a post in the Codeforces UI, save a draft, update it a few times, think it's ready, and publish it. I open my brand new post and see:
> ### Totally not a useless post
> By [user:purplesyringa,2024-09-24], history, 3 days ago, In English
Okay, wait, "3 days ago"? Oh, right, that's when I saved the first draft.
Now people open the post, see "3 days ago", see 0 upvotes, and what do they think? That's right, they think it's garbage because no one's voted positive, so they skip the article.
Maybe not everyone's like tha...
not *against* the spirit of CF per se, but the home page isn't a good place
for them., , making what the button "Delete" does clear, and adding "Delete translation".
IfCF ever wants to, Please bring CF to the same level. My proposition is to:
72.
[Tutorial] Diameter of a tree and its applications Hello everyone,<br>
finding the diameter is one of the most frequent ways to solve problems about trees. In this tutorial we will see how to find a diameter and some of its properties, and we will use them to solve some problems of increasing difficulty.<br>
The first part of the tutorial is quite basic, so feel free to skip it and jump to the problems if you already know the concepts.
Target: rating $[1400, 2300]$ on CF<br>
Prerequisites: basic graph theory, greedy
The diameter
------------------
Given an unweighted tree, let's define $\text{dist}(a, b) =$ the number of edges in the simple path $a \rightarrow b$.
A diameter of the tree $a \rightarrow b$ is the longest path, i.e., the one that maximizes $\text{dist}(a, b)$ over all pairs of nodes. If there are multiple diameters, let's pick any of them.
The same definition is valid for a weighted tree with nonnegative weights (with $\text{dist}(a, b) =$ the sum of the weights of the edges in the simple path $a \rightar...
Target: rating $[1400, 2300]$ on CF
Prerequisites: basic graph theory, greedy
73.
A Beautiful Technique for Some XOR Related Problems ## Inspiration
I'm very excited about this blog, as it took me quite a lot of effort and scavenging through the internet to completely grasp the concept of this technique(That's probably because I have almost zero knowledge in Linear Algebra or I'm just plain dumb). So I feel like I genuinely conquered a challenge, and I really want to share it with someone. But there's no way my CP friends circle will believe it, they'll think I'm just trying to show off :P
So here I am, sharing it on CF. I also created a [personal blog](https://drschwad.github.io/), so that if I ever feel like sharing something again(not only about CP), I can write a blog there. I also added this same post [there](https://drschwad.github.io/2019-08-06-z2-space-xor-trick/), you can read it there if you prefer dark theme. I'll be pleased to hear any thoughts on the blog or if I can improve it in some way ^\_^
## Introduction
Since it concerns Linear Algebra, there needs to be a lot of formal stuff going on ...
So here I am, sharing it on CF. I also created a [personal
blog](https://drschwad.github.io/), so, This is my first take on writing tutorial blogs on CF. I hope it'll be of use
to the community.
74.
MIT Informatics Tournament 2025-26 Winter Contest **UPD 4:** The contest problems and editorials are available in Codeforces gym:
[MITIT Winter 2025-26 Advanced Team Round](https://codeforces.me/gym/106250)
[MITIT Winter 2025-26 Beginner Round](https://codeforces.me/gym/106251)
[MITIT Winter 2025-26 Advanced Individual Round](https://codeforces.me/gym/106249)
**Editorials:** [Team Round Editorials](https://codeforces.me/gym/106250/attachments/download/34709/MITIT_Winter_2025_2026_Team_Editorials-1.pdf).
**UPD 3:** All contests have ended. Thank you all for participating! We will release the rankings and editorials soon.
**UPD 2:** The team contest has officially started. You can view the [live scoreboard](https://mitit.org/Contest/ViewScoreboard/advanced-team-25-26) here.
**UPD 1:** The contest has officially started. Good luck and have fun!
Please join our Discord server [here](https://discord.gg/gmTUzcezv4) for the latest information!
Hello Codeforces!
We ( [user:ETK.,2025-11-06], [user:flame...
** for Advanced (CF rating).
75.
A Highly Experimental Training Plan for Beginners Very loosely inspired by some comments on [Thoughts on Reaching Cyan?](https://codeforces.me/blog/entry/101561) and [my recent AMA](https://codeforces.me/blog/entry/118845).
Until now, I've avoided telling beginners to practice math before working on competitive programing problems because I haven't figured out a helpful way of doing so. This post is my attempt at telling grays to train math first in a way that might lead to improvement without an absurd time commitment.
## Introduction
This section is largely motivation for why I'm proposing this training plan. If you just want to see the instructions for the training plan I'm proposing, you can skip this part. The first subsection in particular is not especially relevant to the rest of the post, but I wanted to have some documentation explaining why I still think the traditional "just solve problems" advice is good.
### The Conventional Advice on Improvement
I'm frequently asked for advice on how to improve at co...
before I started CF, but I was still a relative beginner when I signed up for
CF, so I think the larger
76.
World Finals 2017 statistics and my prediction **1. Tsinghua** (Win: 40%, Gold: 90%, Medal: 100% after some rounding)
- CF max: 3441, 3203, 2967 (agg = 3516)
- Open Cup: 1, 1, 11, 9 (median = 5)
- China Final: 1
- MIPT: 1, 1, 1, 2, 4, 1, 4
**2. Warsaw** (Win: 15%, Gold: 75%, Medal: 95%)
- CF max: 3122, 2962, 2826 (agg = 3237)
- Open Cup: 2, 3 (median = 2.5)
- Petrozavodsk: 4, 2, 4, 3, 2, 2, 1, 2
**3. SPb ITMO** (Win: 15%, Gold: 50%, Medal: 90%)
- CF max: 2880, 2870, 2860 (agg = 3104)
- Open Cup: 7, 3, 2, 1, 13, 1, 2, 1, 3, 7, 13, 8, 8, 12, 3, 2, 1, 12 (median = 3)
- Petrozavodsk: 4, 1, 3, 1, 6, 5, 3, 7, 5
- MIPT: 6, 2, 3, 1, 5, 2, 1
**4. SPb SU** (Win: 10%, Gold: 40%, Medal: 85%)
- CF max: 3170, 2951, 2936 (agg = 3281)
- Open Cup: 9, 21, 18, 5, 3, 4, 9, 9, 6, 1, 2, 2, 7, 1, 4, 23, 20, 4 (median = 5.5)
- Petrozavodsk: 7, 3, 6, 5, 8, 1, 4, 5, 1
- MIPT: 2, 4, 2, 3, 1, 10, 8
**5. Seoul** (Win: 5%, Gold: 40%, Medal: 85%)
- CF max: 3174, 2919, 2760 (agg = 3250)
- Open Cup: 4, failed, 4 (median = 4)
...
**1. Tsinghua** (Win: 40%, Gold: 90%, Medal: 100% after some rounding) - CF
max: 3441, 3203, - CF max: 2543, 2367, 2327 (agg = 2667) - Open Cup: 6, 13, 9, 6, 13, 18 (median
= 11) - China, - CF max: 2707, 2609, 2582 (agg = 2874) - Open Cup: 26, 3, 18, failed, 21, 12,
10, 20, 26, 14, - CF max: 2721, 2280, 1516 (agg = 2745) - Open Cup: 15, failed, 9, failed, 16,
4, 17, 11, 30, 18, - CF max: 2761, 2637, 2273 (agg = 2870) - Open Cup: 2, 17, failed, 7 (median =
12) - MIPT: 5, 5, - CF max: 2762, 2694, 2424 (agg = 2903) - Open Cup: 29, 5, 10, 12, 14, 29
(median = 13) - MIPT, - CF max: 2772, 2341, 2220 (agg = 2808) - Open Cup: 11, 15, 21, 10, 2 (median =
11), - CF max: 2825, 2625, 2466 (agg = 2943) - MIPT: 7, 13, 8, 11, 9, 13, 12, - CF max: 2880, 2870, 2860 (agg = 3104) - Open Cup: 7, 3, 2, 1, 13, 1, 2, 1, 3,
7, 13, 8, 8, 12, 3, - CF max: 2900, 2637, 2540 (agg = 2981) - Open Cup: Equivalent to 2.5 in NAIPC, - CF max: 2920, 2566, 2338 (agg = 2965) - Open Cup: 21, 3 (median = 12) - China
Final: 6, - CF max: 2925, 2908, 2703 (agg = 3100) - Open Cup: Equivalent to 12.5 in NAIPC
- Barcelona: 3, 2, - CF max: 2928, 2757, 2756 (agg = 3063) - Open Cup: 16, 29, 27, 28, 7, 24, 17,
15, 9, 15, 17, 3, 9, - CF max: 3037, 2850, 2578 (agg = 3127) - Open Cup: 6, 18, failed, 3, failed,
9, 18, 7, 24, 15, 14, - CF max: 3073, 2429, 2006 (agg = 3082) - Open Cup: 6, 8, 16 (median = 8) -
MIPT: 14, 9, 10, 16, - CF max: 3122, 2962, 2826 (agg = 3237) - Open Cup: 2, 3 (median = 2.5) -
Petrozavodsk: 4, 2, 4, - CF max: 3170, 2951, 2936 (agg = 3281) - Open Cup: 9, 21, 18, 5, 3, 4, 9, 9,
6, 1, 2, 2, 7, 1, 4, - CF max: 3174, 2919, 2760 (agg = 3250) - Open Cup: 4, failed, 4 (median = 4), - CF max: 3441, 3203, 2967 (agg = 3516) - Open Cup: 1, 1, 11, 9 (median = 5) -
China Final: 1
77.
[CONTEST OVER] MIT Informatics Tournament 2024 Spring Invitational **Update 2**: The Qualification Round has **officially concluded**! A huge thank you to our contestants for joining us and supporting this new initiative! Congratulations to the top 16 finalists who are advancing to the finals round (along with many more as we continue to confirm people's availability)!
1. [user:Rewinding,2024-04-24]
2. [user:ksun48,2024-04-24]
3. [user:Petr,2024-04-24]
4. [user:Geothermal,2024-04-24]
5. [user:mango_lassi,2024-04-24]
6. [user:hank55663,2024-04-24]
7. [user:ecnerwala,2024-04-24]
8. [user:MiFaFaOvO,2024-04-24]
9. [user:Kevin114514,2024-04-30]
10. [user:A_G,2024-04-30]
11. [user:Adam_GS,2024-04-24]
12. [user:Tourist,2024-04-24]
13. [user:FlowerOfSorrow,2024-04-24]
14. Matthew Allan
15. [user:uwu,2024-04-24]
16. [user:TLE,2024-04-24]
At this time, the scoreboard has been finalized, and you can upsolve our problems / watch our video tutorials with the following links:
- Finalized Scoreboard: [https://mitit.org/Contest/ViewScoreboar...
://mitit.org/Contest/ViewScoreboard/qualification-2024) - Upsolving CF Gym:
[https://codeforces.me/gym
78.
Educational Codeforces Round 103 Editorial [problem:1476A]
Idea: [user:vovuh,2021-01-30]
<spoiler summary="Tutorial">
[tutorial:1476A]
</spoiler>
<spoiler summary="Solution (adedalic)">
~~~~~
#include<bits/stdc++.h>
using namespace std;
int main() {
int t; cin >> t;
while(t--) {
long long n, k;
cin >> n >> k;
long long cf = (n + k - 1) / k;
k *= cf;
cout << (k + n - 1) / n << endl;
}
return 0;
}
~~~~~
</spoiler>
[problem:1476B]
Idea: [user:adedalic,2021-01-30]
<spoiler summary="Tutorial">
[tutorial:1476B]
</spoiler>
<spoiler summary="Solution (adedalic)">
~~~~~
#include<bits/stdc++.h>
using namespace std;
typedef long long li;
const int INF = int(1e9);
int main() {
int t; cin >> t;
while(t--) {
int n, k;
cin >> n >> k;
vector<int> p(n);
for (int i = 0; i < n; i++)
cin >> p[i];
li x = 0;
li pSum = p[0];
for (int i = 1; i < n; i++) {
x = max(x, (100ll * p[i] - k * pS...
long long cf = (n + k - 1) / k; k *= cf;
79.
Codeforces Round 1067 (Div. 2) <img src="/predownloaded/cc/cf/cccf1d602af8c092d82d3bc34ca8a2a4ac0582e8.gif" style="width:180px; float:right; margin-left:10px;">
Good day Codeforces ✨
----------------------------
We are excited to invite you all to [contest:2158], starting at [contest_time:2158].
This round features **6 problems** to be scored in **2 hours**, with a few problems including multiple parts. All tasks were authored and prepared by [user:koderkushy,2025-11-28] and me. This round will be **rated for all participants with rating below 2100**. Take a look at all the problems, we hope you enjoy the challenge! 🚀
We extend many thanks to our team for helping make this round possible:
- [user:abc864197532,2025-11-28] for his excellent coordination.
- [user:KAN,2025-11-28] for preliminary review.
- Testers lineup: [user:Dominater069,2025-11-28], [user:dreamoon_love_AA,2025-11-28], [user:Fysty,2025-11-28], [user:amano_hina,2025-11-28], [user:_istil,2025-11-28], [user:__baozii__,2...
Announcement of Codeforces Round 1067 (Div. 2)
80.
On problem 2075B (ER176 B) and why it exists [SPOILERS] So, if you paricipated in ER 176 and got WA2 on problem B, you might think this is a terrible problem, maybe one of the worst on CF. Let me try to explain why it was created, and maybe it will change your opinion about it (or maybe it won't).
This problem has an obvious solution: you have to maximize the sum of the first $k$ chosen elements and the last element you paint, so the sum of $(k+1)$ elements of the array. So, let's take $(k+1)$ greatest elements, add them up, and we get the answer. It even passes the sample test case, but it is wrong.
<spoiler summary="Why it is wrong">
If $k=1$, there is only one "starting" element. So, the last element you paint must be on the border (the $1$-st or the $n$-th element of the array).
If $k>1$, the greedy solution actually works since any element between two elements which were initially chosen can be the last element. So, out of $(k+1)$ maximums, you can initially paint them all blue, except for the middle of them. But if $k=1$, yo...
problem, maybe one of the worst on CF. Let me try to explain why it was
created, and maybe it will change, But I still think that such problems should be on CF, especially because most
other problems are
81.
Codeforces Round #250 Happy Children's Day everyone. What should you do to celebrate the special holiday?
Of course, another Codeforces Round is your best choice!
There is a child who is our friend. Now he has faced a lot of algorithm problem, could you please help him?
We invite you to participate in Codeforces Round #250, which will take place at [17:00 MSK on 6.1 — Children's Day](http://www.timeanddate.com/worldclock/fixedtime.html?day=1&month=6&year=2014&hour=17&min=0&sec=0&p1=166). This round will be held in both divisions.
Note that the starting time of this round is quite unusual.Why? Because it's yet another CF Round held by Chinese! We prepared many interesting problems for you.
Are you getting excited? Don't miss this round!
The problems were prepared by [user:Delayyy,2014-05-31], [user:Picks,2014-05-31] and me. This is our first Codeforces round~~~~.
Many thanks to [user:Gerald,2014-05-31], wh...
Note that the starting time of this round is quite unusual.Why? Because it's
yet anotherCF Round
Announcement of Codeforces Round 250 (Div. 2)
Announcement of Codeforces Round 250 (Div. 1)
82.
CF-Predictor news Hello all!
I have some news to share about CF-Predictor (a service that allows you to view rating changes before official results). This extension [was launched](https://codeforces.me/blog/entry/50411) almost 6 years ago. Number of users has grwon from a couple hundreds to nearly 50k! I'm very proud that this service is used by so many people!
One of the cool features of mine extension is that it is friendly to codeforces API :) All the request to codeforces are sent from just one backend server. Therefore, users don't overload codeforces with extra API calls.
I was using Heroku as a cloud provider to run my servers. Unfortunately, they are closing their free accounts on Nov 28th. So, I decided to take down my servers on heroku. Thus, starting from 2022-11-27 [cf-predictor-frontend.herokuapp.com](https://cf-predictor-frontend.herokuapp.com) and [cf-predictor.herokuapp.com](https://cf-predictor.herokuapp.com) will **not** work.
On a bright side, I though that upsetting 50k ...
CF-Predictor news, VPS and you can continue to use CF-Predictor via [cf
-predictor.wasylf.xyz](http://cf, -27 [cf-predictor-frontend.herokuapp.com](https://cf
-predictor-frontend.herokuapp.com) and [cf, Hello all! I have some news to share about CF-Predictor (a service that allows
you to view, I have some news to share about CF-Predictor (a service that allows you to view
rating changes
83.
Most annoying person on CF in 2025 Continuation of:
- [Most annoying person on CF in 2018](https://codeforces.me/blog/entry/64101)
- [Most annoying person on CF in 2023](https://codeforces.me/blog/entry/124510)
Mandatory introduction copypasta:
> It is that time of the year! Many people are trying to summarize all the things that happened during the year. And since I love to hate people, I want to make a vote for the most annoying person on CF in 2025.
>
> I know that I've been in the top contributors list for too long, so **to vote for a person you should downvote the comment with this person's handle(s)**. Due to the nature of voting through comments on CF, you can vote for many people, and you can also upvote comments if you find given person's comments helpful/funny/positive.
>
> If you care for your contribution but want to nominate a candidate, you can DM me their handle(s), then I will write a comment to vote. Note that I will write that it was you who nominated this person, so it saves your con...
Most annoying person on CF in 2025, - [Most annoying person on CF in 2018](https://codeforces.me/blog/entry/64101)
- [Most annoying, Continuation of: - [Most annoying person on CF in
2018](https://codeforces.me/blog/entry/64101
84.
Поддержите Украину (теперь и с Crowdforces) Я знаю, что много кого здесь волнует война в Украине. Если вы ищете способы помочь, перейдите по этой ссылке: http://linktr.ee/help_ukraine_global. Любая поддержка и помощь имеют значение.
Пожалуйста, помогите остановить кровавого диктатора нашего соседа.
**UPD1:** [Еще один отличный сайт](https://www.standwithukraine.how/)
**UPD2:** [С этим сайтом](https://stop-russia.near.page/) вы можете помочь показать жителям России показать, что на самом деле происходит в Украине. Спасибо [этому](https://codeforces.me/blog/entry/100453) посту.
**UPD3:** Если в вашем CF профиле страной была указана Украина на момент публикации [этого](https://codeforces.me/blog/entry/100459) блога, и у вас еще нет аккаунта Crowdforces, вы можете создать его по этой ссылке: https://nearcrowd.com/crowdforces, и получить 12 Near (что сейчас около 100$).
Вы можете найти более подробные инструкции [здесь](https://docs.google.com/document/d/1q_t_hdfVIYSGlMsOWsz3jhEEaLBfY1Yp/edit). Этот документ на данны...
**UPD3:** If your CF profile had country set to Ukraine at the moment of
[this](https, **UPD3:** Если в вашем CF профиле страной была указана Украина на момент
публикации [этого](https
85.
Problem Topics Good Day to you!
I've been asked to make some topic-wise list of problems I've solved. Even though I couldn't involve all problems, I've tried to involve at least "few" problems at each topic I thought up (I'm sorry if I forgot about something "easy"). I've alredy made such list once anyway I've tried to include more problems now — so here it is:
<spoiler summary="aho">
http://www.spoj.com/problems/ADAJOBS/
URI 2226 (5) //[NICE][NUMBERS][DP]
http://www.spoj.com/problems/SUB_PROB/en/
http://codeforces.me/contest/696/problem/D 8
http://www.spoj.com/problems/AHOCUR/ 5 //Aho-Corassic + DP
https://www.codechef.com/problems/LYRC (5) //Sample aho-brute-force
http://codeforces.me/problemset/problem/346/B //Proposed by [user:bradyawn,2019-08-03]
</spoiler>
<spoiler summary="automat">
6861 [LA] //CYK
UVA 10679 //Suffix Automat
http://www.spoj.com/problems/STRMATCH/ //Suffix Automat — trie might do too
http://www.spoj.com/problems/NSUBST...
CF y)
86.
[Tutorial] Problems about swapping adjacent elements Hello everyone,<br>
problems about swapping adjacent elements are quite frequent in CP, but they can be tedious. In this tutorial we will see some easy ideas and use them to solve some problems of increasing difficulty. I tried to put a lot of examples to make the understanding easier.<br>
The first part of the tutorial is quite basic, so feel free to skip it and jump to the problems if you already know the concepts.
Target: rating $[1400, 2100]$ on CF<br>
Prerequisites: greedy, Fenwick tree (or segment tree)
Counting inversions
------------------
Let's start from a simple problem.
_You are given a permutation $a$ of length $n$. In one move, you can swap two elements in adjacent positions. What's the minimum number of moves required to sort the array?_
#### Claim
The result $k$ is equal to the number of inversions, i.e. the pairs $(i, j)$ ($1 \leq i < j \leq n$) such that $a_i > a_j$.
#### Proof 1
Let $f(x)$ be the number of inversions after $x$ moves.<br>
In one...
Target: rating $[1400, 2100]$ on CF
Prerequisites: greedy, Fenwick tree (or segment tree)
87.
Calling CF Community to Support Rescheduling ACPC **UPD: it's now rescheduled! thank you all for the support.**
Hello Codeforces,
I didn't want to publish this blog during these circumstances, as it's the last thing to care about, but I'm publishing it hopefully ACPC organizers see it.
Last Saturday, we got an email saying that the ICPC world finals, which were supposed to be in 11 days in Sharm El-Sheikh, got rescheduled to a later time. And it was expected that the ACPC, which is supposed to be in 17 days in the same hotel, would get rescheduled too. but apparently the competition is still on its previously scheduled date!! I would like to note that ACPC is scheduled to be in the same hotel as ICPC.
Let's talk about it from many perspectives.
From the travel perspective, sadly while talking I will partition Palestine into the West Bank and Gaza, but we're one nation and stand together with each other.
Firstly, two teams from Gaza are supposed to participate in ACPC this year, and surely, they will not be able to co...
Calling CF Community to Support Rescheduling ACPC
88.
Presenting to you the biggest cheat on Codeforces! Hello all,
I was recently intrigued by this account — [user:Ali_Pi,2018-08-02] because of the notorious similarity of username with this account — [user:Ali.Pi,2018-08-02]..
A short note on [user:Ali.Pi,2018-08-02] — he has copy pasted thousands of problems' codes in the practice section, and climbed to the top of standings page [ [LINK](http://codeforces.me/problemset/standings) ] sometime last year. He had even beat the chinese bots to climb to the top! Talk about being jobless. He used to do virtual contests on every random contest — and copy past every problem's code and be #1 when you open the standing page. For what joy? We will have to ask him for the answer.
Coming back to the [user:Ali_Pi,2018-08-02] account — his recent contest on CF — Codeforces Round #500 (Div. 1) based on EJOI — he had solved three problems — [ [Submission for problem A](http://codeforces.me/contest/1012/submission/40945262) ] [ [Submission for...
Cheating during contests is something that has been happening on CF, but on
such a huge magnitude, The audacity to claim that the contest became a "typing contest" — a term used
byCF
89.
Educational Codeforces Round 70 Editorial [problem:1202A]
Idea: [user:adedalic,2019-08-08]
<spoiler summary="Tutorial">
[tutorial:1202A]
</spoiler>
<spoiler summary="Solution (adedalic)">
~~~~~
fun main(args: Array<String>) {
val T = readLine()!!.toInt()
for (tc in 1..T) {
val x = readLine()!!.reversed()
val y = readLine()!!.reversed()
val posY = y.indexOf('1')
val posX = x.indexOf('1', posY)
println(posX - posY)
}
}
~~~~~
</spoiler>
[problem:1202B]
Idea: [user:adedalic,2019-08-08]
<spoiler summary="Tutorial">
[tutorial:1202B]
</spoiler>
<spoiler summary="Solution (adedalic)">
~~~~~
const val INF = 1e9.toInt()
fun main(args: Array<String>) {
val s = readLine()!!.map { it - '0' }
val cf = Array(10) {Array(10) {0}}
for (i in 1 until s.size)
cf[s[i - 1]][s[i]]++
for (x in 0..9) {
for (y in 0..9) {
val ds = Array(10) {Array(10) {INF + 7}}
for (v in 0..9) {
...
) { if (ds[v][to] > INF && cf[v][to] > 0) { res = -1, fun main(args: Array) { val s = readLine()!!.map { it - '0' } val cf = Array(10
90.
Codeforces Round #715 Editorial <strike>Note: I can't figure out how to place the tutorials inside spoilers. If someone is familiar with how CF spoilers work and can help I would really appreciate it. For now, be warned that the tutorials are visible by default (but everything else isn't)</strike>
**UPD:** I figured out how to use spoilers! Also added implementations for all problems.
Thanks for participating in our contest!
[problem:1509A]
Author: [user:Kuroni,2021-04-13]
<br>
First solve: [user:cfg0d,2021-04-16] at 00:01:02
<spoiler summary = "Hint">
How can you write the condition that $\frac{a_u + a_v}{2}$ is an integer in a more useful way? Think of the parities.
</spoiler>
<spoiler summary = "Tutorial">
[tutorial:1509A]
</spoiler>
<spoiler summary = "Comments from the authors">
</spoiler>
[Implementation](https://codeforces.me/contest/1509/submission/113263196)
[problem:1509B]
Author: [user:Ari,2021-04-13]
<br>
First solve: [user:bl...
with how CF spoilers work and can help I would really appreciate it. For now,
be warned that the
Tutorial of Codeforces Round 715 (Div. 1)
Tutorial of Codeforces Round 715 (Div. 2)
91.
My Summer CP School, rating 1000-2200 Hi, I'm going to teach competitive programming classes this Summer, starting with a 4-week batch to see how it goes.
I hope to make 2 or 3 groups of ~12 students, each group aimed at some CF rating.
We will use problems from Codeforces, CSES, Atcoder, Polish olympiad (only higher group), and my modifications and follow-ups.
There will be homework, some to be discussed next lesson.
- Dates: June 24 to July 19 (4 weeks)
- Time: morning-noon in Europe
- 2-3 lessons per week, 1.5h or 2h each
- Price: around 20 USD per hour, 260-440 USD total per person
- Platform: Discord and/or Google Meet
Please fill [this form](https://forms.gle/v1q7qZKU1YnNWRbaA) to get a 5% early-bird discount.
I will use the results to choose and announce groups (difficulty and frequency) in 1-2 weeks.
Feel free to ask questions in the comments, or DM me.
<spoiler summary="regarding the idea of USACO classes">
I already mentioned [here](https://codeforces.me/blog/entry/129009) my plan to do USAC...
=30), CF max rating 1000-1400 (time in CEST, Poland) 2.
[10:00-11:30](https://www.timeanddate.com
92.
My story at ACM ICPC World Final 2016 My team (me + [user:flashmt,2016-05-20] + [user:nguyenhungtam,2016-05-20]) was probably never noticed by anyone, but we were actually aiming for medal. And I think we performed ok in the final, ending up at #15, still need some more luck & brain to win a medal. Anw, I decided to share my story. Btw if you are Vietnamese, you can also read [this](https://www.facebook.com/notes/thanh-trung-nguyen/t%C3%B3m-t%E1%BA%AFt-acm-world-final-2016/1157851804228014?pnref=story).
Back story
==========
In the ACM history, a Vietnamese team has never won any WF medal. 5 years ago, team of [user:ktuan,2016-05-20] (only Vietnamese red target on Topcoder) had high hope, but ended up at #17. 4 years ago, team of [user:ConanKudo,2016-05-20] also had high hope, but messed up in the first 3 hours and ended up at #17.
So I really wanted to win medal. And I've been planning for this World Final for several years..
Team formation
==============
I've chosen [user:flashmt,2016-05-20] as teamm...
After this WF, probably I'll partially retired (probably still participate in CF
rounds & big
93.
Changing your rank to lower one is mainstream above 1579 <spoiler summary="Disclaimer">
All data presented is actual as of 2024/12/28, that is, before Good Bye 2024.
Also, since Ratings page only shows active users, I decided not to count any others.
I apologize to anyone I have not included.
</spoiler>
Looking at the top-10 users and Recent Actions bar right now, two things become apparent:
1. Since LGMs cannot change their rank to a higher one, more people in, say, top-100 have changed their rank to a lower one than to a higher one.
2. Codeforces-wide, more people have changed their rank to a higher one than a lower one.
So there has to be an exact cut-off point, above which the same number of people have changed their rank to either side.
I decided to do some scraping to find out exactly where it is.
Lo and behold:
<img src="https://github.com/maslinnik/cf-magic/blob/master/images/by_position.png?raw=true" alt="Rank changes (leaderboard position)">
Of the top 15k users, about tenth changed their rank to a higher ...
94.
Codeforces Wrapped 2022 — Поделитесь результатами года! С Новым Годом, Codeforces!
Я большой фанат статистики и еще больший — сайта Codeforces. Решив объединить эти свойства, я создал сервис для того, чтобы дать всем пользователям этого чудесного сайта возможность посмотреть свою годовую статистику.
Основными моими целями было сделать такую карточку с данными, чтобы:
- она содержала полезную статистику;
- она не была перегружена информацией;
- ей было легко поделиться.
Чтобы этого добиться, я выкинул некоторую статистику, которая оказалось слишком скучной, которая занимала слишком много места <s>или которую было слишком тяжело реализовать в рамках CF API</s>. Надеюсь, вам понравится результат!
Сервис довольно интуитивный в использовании. Введите свой хэндл (если меняли, то введите тот, который был в течение года), получите карточку с результатом и поделитесь ей с миром, нажав "open as image".
<spoiler summary="Дисклеймер">
Я не профессиональный веб-разработчик, даже не назвал бы себя опытным. С помощью друзей из k...
were hard to implement within the limitations of CF API. I find the final
choice of data really fun, занимала слишком много места или которую было слишком тяжело реализовать в
рамкахCF API
95.
Dirichlet convolution. Part 1: Fast prefix sum computations Hi everyone!
Suppose that you need to compute some sum of a number-theoretic function that has something to do with divisors:
$$\begin{gather}
\sum\limits_{k=1}^n \varphi(k) = ? \\
\sum\limits_{k=1}^n \sum\limits_{d|k} d^2 = ?? \\
\sum\limits_{x=1}^n \sum\limits_{y=1}^x \gcd(x, y) = ?!?
\end{gather}$$
As it turns out, such and many similar sums can be computed with Dirichlet convolution in $O(n^{2/3})$, and in this article we will learn how.
Let $f(n)$ and $g(n)$ be two [arithmetic functions](https://en.wikipedia.org/wiki/Arithmetic_function). Let $F(n)$ and $G(n)$ be their prefix sums, that is
$$\begin{matrix}
F(n) = \sum\limits_{i=1}^n f(i), & G(n) = \sum\limits_{j=1}^n g(j).
\end{matrix}$$
We need to compute a prefix sum of the [Dirichlet convolution](https://en.wikipedia.org/wiki/Dirichlet_convolution) $(f * g)(n)$. In this article, we will consider some general methods, and show how to do so in $O(n^{2/3})$ if we can compute prefix sums of $F(n)$ and $G(n)$ in ...
96.
Educational Codeforces Round 62 Editorial [problem:1140A]
<spoiler summary="Tutorial">
[tutorial:1140A]
</spoiler>
<spoiler summary="Solution (adedalic)">
~~~~~
#include<bits/stdc++.h>
using namespace std;
int n;
vector<int> a;
inline bool read() {
if(!(cin >> n))
return false;
a.resize(n);
for(int i = 0; i < n; i++) {
cin >> a[i];
a[i]--;
}
return true;
}
inline void solve() {
int cnt = 0, pos = 0;
while(pos < n) {
cnt++;
int mx = pos;
while(pos < n && pos <= mx) {
mx = max(mx, a[pos]);
pos++;
}
}
cout << cnt << endl;
}
int main() {
#ifdef _DEBUG
freopen("input.txt", "r", stdin);
#endif
if(read()) {
solve();
}
return 0;
}
~~~~~
</spoiler>
[problem:1140B]
<spoiler summary="Tutorial">
[tutorial:1140B]
</spoiler>
<spoiler summary="Solution (Roms)">
~~~~~
#include <bits/stdc++.h>
using namespace std;
int t, n;
string s;
int main(){
cin >> t;
for(int tc = 0; tc < t; ++tc){
cin >> n >> s;
int res = n - ...
int calcSeg(int l, int r) { if(r >= sz(curArray)) { int len = r - l - 1, cf = 1
97.
CF New Chrome Extension (Userscript): CF Get Problems Hello Everyone!
Recently I was exploring [Codeforces APIs](https://codeforces.me/apiHelp) and thought that there is a need for an extension that shows problem difficulty as well as all the contest problems links together _(as usually it takes some time switching from status/submission/problem page to dashboard to select a desired problem)_.
So, well here it is!
With the help of this extension, one can go through any problem to any other problem easily (also can go to a problem from status/submission/hack pages to the desired problem).
You can try this extension.
##### Steps to follow (for the installation)
Right now I haven't deployed/uploaded it on either Chrome or Firefox but you can install it locally!
You just have to clone [this repo](https://github.com/dhanrajchaurasia/CF-Get-Problems) or can download via zip and load/unpack the folder over the chrome/firefox extension (with developer mode ...
CF New Chrome Extension (Userscript): CF Get Problems, clone [this repo](https://github.com/dhanrajchaurasia/CF-Get-Problems) or can
download via zip and load, locally! You just have to clone [this
repo](https://github.com/dhanrajchaurasia/CF-Get-Problems) or, , **UPD4:** The extension is now
[live](https://addons.mozilla.org/en-GB/firefox/addon/cf-get, /CF-Get-Problems)).
98.
About Codeforces rules, conventions and assumptions I don't feel like my attention-seeking senses are fulfilled with just comments, so I think it makes more sense to write a blog about it. Codeforces has evolved over the years and I think it might be a good move to talk about some relicts of the past that are still there, or even possibly do something about them. I'd like to note, that it's possible that there've been some blogs about this topic from the admins and I've missed them and lack information because of that. In this case, please pardon me.
After the first two Spectral::Cup rounds I've commented on things that in my opinion are somehow harmful.
The first comment was [this](https://codeforces.me/blog/entry/153179?#comment-1361697) one. It was mostly about the solution being judged twice on the same tests. I'm not much against it, as it seems to be well defined in theory: you have to pass both times to get AC (where the second time it might include extra tests), which is fine — not perfect, but ok, rules are rules. ...
solution from the previous paragraph happened because of cf team wanting to be
kind, but that couldn't
99.
CodeCraft-21 and Codeforces Round #711 (Div. 2) Hello all!
We are proud to invite you to **CodeCraft-21**, which takes place on [contest_time:1498]. It is rated for all Div2 participants (rating under 2100).
This contest comes under **Threads'21**, as a part of our annual techno-cultural fest [Felicity, IIIT Hyderabad](https://felicity.iiit.ac.in). We have **special prizes for Indian participants**!
<img src="/predownloaded/8e/0a/8e0a76faf6acc96ba6ffb9b873136259066b5211.png" width="450px">
You will be given **six problems** to solve in a duration of **two hours**.
We have tried our best to keep clean statements, useful samples with good explanations, and strong pretests! We have written step-wise editorials, and will also release video editorials!
We would like to thank these amazing people for helping make the contest happen:
- [user:bleddest,2021-03-27] for great coordination of our round
- [user:ninja_28,2021-03-27], [user:dixitgarg,2021-03-27], [user:ltc.groverkss,2021-03-27] and myself for amazing problem...
These ranks are determined from the combined unofficial ranklist. We are
coordinating with theCF
Announcement of CodeCraft-21 and Codeforces Round 711 (Div. 2)
100.
[Tutorial] Catalan Numbers and Catalan Convolution Catalan Numbers is a well known sequence of integers that appear in combinatorics, there is a chance that you might have run into such counting problems and you might have even solved them with DP without realizing that they are a known sequence. Here is a problem to get us started.
### Binary Trees Count
Find the count of different binary trees with $n$ nodes, we'll denote to that by $C_n$. This can be solved by observing the substructure of the problem:
From the root, the left subtree $a$ can be _any_ binary tree of size $x$, the right subtree $b$ can be _any_ binary tree of size $y$, such that $x+y=n-1$ (as we took one node as the root). With the base case at $C_0=1$ as the empty tree is a valid tree of size $0$. This directly gives us the answer by the following recurrence relation: (Segner's recurrence relation)
$$ C_{n+1} = \displaystyle\sum_{i=0}^{n}{C_i C_{n-i}}, n \geq 0; C_0=1 $$
...
101.
Don't let CF Ratings be a Joke! Ok,someone needs to say this!
I support all the new changes in the rating system except for one, which says divisions would be assigned by displayed ratings. This is a disaster! Please, change it!
Since, it's gonna take 6 contests for stabilisation of ratings,people whose true rating > 1600 would continue to give Div-3 and Div-4 contests. By the time, the rating stabilises, there are gonna be many undeserving Candidate Masters and Masters.
Trust me, it's gonna rain CMs! I believe majority of such false candidate masters would not give div-2/div-1 contests after that due to fear of fall. This would lead to degradation of a Candidate Master on Codeforces (just like 5-star coder on Codechef).
As Codechef has lost the credibility of their ratings due to Long Challenges, Codeforces is on the way to line up behind them, with the introduction of div-4 and now this.
It's my sincere request to [user:MikeMirzayanov,2020-05-25] to look into this matter(at least don't let 1600+ con...
Don't let CF Ratings be a Joke!, let 1600+ contestants participate officially in div-3) and stop CF ratings
from being a joke!
102.
Чтобы ускорить ввод-вывод в 5 раз, нужно всего лишь... ...воспользоваться простой советской содой:
```cpp
#include <iostream>
int main() {
std::ios_base::sync_with_stdio(false);
std::cin.tie(nullptr);
// Ваш код тут
int n;
std::cin >> n;
long long sum = 0;
for (int i = 0; i < n; i++) {
int x;
std::cin >> x;
sum += x;
}
std::cout << sum << std::endl;
}
```
Это ускорит код настолько, что вам в жизни больше не придется думать о скорости ввода-вывода. Ставьте классы и подписывайтесь на мой канал!
[cut]
<!-- -->
---
Ага, размечтались. Это не так работает. <s>Мы строили культуру и...</s> У нас все еще используется `<iostream>`, а значит, в коде появляется виртуальное наследование и все вытекающие из этого деоптимизации. Даже если вы нашли на задворках интернета сниппет "быстрый-ввод-вывод-от-васяна", вы все равно опираетесь на блочно буфферизованный вывод. (К тому же никакая из этих библиотек и знать не знает, что такое эти ваши флоаты.) В большинст...
103.
Does Codeforces need a moderation system? In the recent months, the number of trash blogs on Codeforces has risen. Thankfully, they are mostly being ignored, and are very often recognizable by their title and/or author handle, but they still show up in the Recent Actions, making navigation through actually meaningful and good content like tutorials/contest invitations/etc harder. And very often, it's the same person posting one useless blog after another, not getting banned even after five or six instances. Personally, I am sick of those blogs, and I don't think I'm the only one.
I understand why they don't get banned. Mike, Gera Nazarov and other people responsible for the platform are very busy maintaining the testing system, catching cheaters, improving Polygon and doing other work which lets Codeforces continue running and improving. They don't have time for digging through everything that gets posted, and that's perfectly understandable.
So, I think we need some other people who can do this. Maybe a crew of some uni...
104.
Codeforces Round #356 Hello Codeforces.
The CF Round \#356 will happen on 8-th June ([exact time](http://www.timeanddate.com/worldclock/fixedtime.html?day=8&month=6&year=2016&hour=19&min=35&sec=0&p1=166)).
You will get five problems to solve in two hours.
The round is rated.
I encourage you to read other problems if you don't like or can't solve something.
The scoring distribution will be announced.
[user:MikeMirzayanov,2016-06-06] and [user:GlebsHP,2016-06-06] make the round possible.
Also, thanks for [user:Stonefeang,2016-06-06], [user:kostka,2016-06-06], [user:johnasselta,2016-06-06], [user:AlexFetisov,2016-06-07] and (more to be added?) for their amazing help.
And I want to thank my girlfriend because there would be no Limak without her.
It's my first standard round.
Still, you should get nice interesting problems.
You will meet Limak, who is usually a little polar bear.
Here he is, preparing one of problems.
Announcement of Codeforces Round 356 (Div. 2)
105.
You can do it, too! Introduction
-------------------------------
So, I set myself [that challenge](https://codeforces.me/blog/entry/81383) to retire as a red coder before 24.09.2020... and I failed **miserably**. At least I am less delayed than most civil infrastructure projects.
This blog will be a recap of this _short_ challenge with a mixture of some tips on practicing (for all levels!), some random life advice and, of course my bad sense of humour. It is dedicated to all those who aren't wonderkids, go from failure to failure and feel like banging their head into a wall during most of their competitive programming journey. Whatever your goal is, I'd like to strengthen your beliefs — you can do it!
How it all started
------------------
My skill level has been in constant stagnation for years. I've always thought that I'm just not talented enough to be a grandmaster. Although I've been red twice in the past, it was quite lucky and the next contests were solid $-137$ and $-189$ so th...
started doing virtual CF contests and upsolving almost every day. That was
quite a challenge, because I, The truth is that although older CF contests were very good practice for ICPC,
they are not a very
106.
Egyptian team is likely not going to IOI. Again. Hello Codeforces,
I'm a member of the Egyptian IOI 2022 team, and I'm not sure yet if we are going to IOI. Historically, the Ministry of Communication and Information Technology (MCIT) sponsored the Egyptian team to travel to IOI. However, this year, there is no confirmation from the MCIT's side at all. Our tickets are not booked until now, and MCIT doesn't either confirm or deny that they will sponsor us this year.
Now, there are fewer tickets left, and the cost of traveling increases as we approach IOI. We all have been practicing really hard for IOI for the past few months. Now, there is very little time for us to make alternative plans to participate onsite.
Note that MCIT provides training and sponsors the team every year so we expected more from the MCIT side. AASTMT has supported us with a part of ticket expenses
In an attempt to collect the remaining part, we created a crowdfunding campaign. Please donate [here](https://gogetfunding.com/iiot-fundraising/).
UPD: W...
UPD: We've reached the crowdfunding campaign goal. Thanks to CF community.
107.
Codeforces Round #317 <img src="http://aimfund.ru/bitrix/templates/aimfund/images/logo.png" style="width:120px; height:179px; float: right;margin: 0 1.0em 1.0em 1.0em;" />
Привет, Codeforces!
<a href="http://www.timeanddate.com/worldclock/fixedtime.html?msg=Codeforces+Round+%23317+%5BAimFund+Thanks-Round%5D&iso=20150822T1930&p1=166&ah=2">22 августа, в субботу, в 19:30 MSK</a> состоится 317 раунд Codeforces.
Раунд подготовили для вас сотрудники компании AimFund: [user:Kostroma,2015-08-19], [user:AlexDmitriev,2015-08-19], [user:yarrr,2015-08-19], [user:gchebanov,2015-08-19], [user:ArtDitel,2015-08-19], [user:Baz93,2015-08-19] и [user:zeliboba,2015-08-19].
В каждом из дивизионов участникам будет предложено пять задач и два часа на их решение. Разбалловка будет статическая.
Благодарим Михаила Мирзаянова ([user:MikeMirzayanov,2015-08-19]) за замечательные платформы Polygon и Codeforces, координатора задач Codeforces Макса Ахмедова ([user:Zlobober,2015-08-19]) и Марию Белову ([user:Delinur,2015-08-...
. С этого раунда на CF начинается серия thanks-раундов, которые посвящаются
людям и компаниям
Announcement of Codeforces Round 317 [AimFund Thanks-Round] (Div. 1)
Announcement of Codeforces Round 317 [AimFund Thanks-Round] (Div. 2)
108.
[GYM] 100 Easy Problems (Bootcamp) Target audience: newbies and pupils (rating up to 1400).
Group link: https://codeforces.me/group/yg7WhsFsAp/contests (hit "join" on the right).
Hi.
Enjoy a series of 8 problem lists for beginners.
The example topics are strings, arrays, math, and binary search.
You are allowed to discuss anything with others, or just look up solutions online.
There are also 3 exams, each recommended for a 2-hour individual virtual participation.
Use the displayed order, e.g. take exam 1 after day 3.
It all should take you 2 weeks of intense bootcamp-like work (or a few months if you take your time).
The problems were originally used two years ago in a Saudi Arabia camp.
It's a mix of around 70 existing CF problems and 30 new original problems, mainly prepared by [user:kostka,2021-11-18], with some help from me and [user:mustafabar,2021-11-18]. I asked them for permission to publish everything.
I will put hints to some problems in this blog (or in the group? not sure). Expect a few v...
existing CF problems and 30 new original problems, mainly prepared by
[user:kostka,2021-11-18], with
109.
Please change the score of hacking in CF rounds Hi!
In this blog I would like to humbly suggest a change in how hacking is calculated in CF rounds. I've talked to some of my friends and they agree that it feels a little unfair that someone who solved more problems scores less than someone who hacked more. A great example from where this happens is from the top positions (and other positions too) in today's round: [Codeforces Round #482 (Div. 2) ](http://codeforces.me/contest/979/standings).
I have two main suggestions on how to calculate hacking points in a way that I consider fairer. The specific way may vary, but the key points are:
1) **The more you successfully hack a certain problem, the less you should score with it.** This means that the first hack on problem A should get you 100 points, but the second 50, the third 25 and so on (or something like this, maybe keep 50 for all the following hacks starting with the second one). I feel that a decreasing function for this is fairer because, in most cases, when you hack a...
Please change the score of hacking in CF rounds, In this blog I would like to humbly suggest a change in how hacking is
calculated inCF rounds
110.
An IDE for competitive programming: fetch testcases from websites, test on testcases in one click, and more When solving CP problems, are you tired of copy-pasting the testcases, and compare the outputs with the answers by eyes?
Today I'll recommend a free-software IDE for those who want to automate everything in competitive programming.
[](https://github.com/coder3101/cp-editor/releases/latest)
[](https://github.com/coder3101/cp-editor/releases)
CP Editor began from running testcases in one click, and has been grown into an IDE with many features, like parsing sample testcases from websites and submitting to Codeforces inside the IDE. It's cross-platform and lightweight, and most important, specially designed for competitive programming.
You can learn about more features on the [official website](https://cpeditor.github.io/).
It's also recommended...
**UPD: You should use CF Tool 0.9.0 as for now, we will support CF Tool 1.0.0
soon.**
111.
LGMSniper.com: See which LGMs you have beaten! LGM Sniper
==================
I just created a cool CF tool called [LGM Sniper](https://wumbogames.com/CompetitiveProgramming/LGMSniper). It looks at scoreboard data from every Div1 and Div1+Div2 round ever and tells you which LGM's you have beaten, how many times you have beaten them, and which rounds you beat them in. You can also compare yourself to your friends to see which one of you wins more! If you want to compare yourself with some friends, check the 'show ex-lgms' box and enter the friend you want to compare to as Handle #2, and they will show up on the bottom.
You can try it out for yourself at [lgmsniper.com](https://wumbogames.com/CompetitiveProgramming/LGMSniper), or in the competative programming section of [Wumbo Games](https://wumbogames.com/), if you would prefer. Tooltips show the name of the rounds you won in (hover your mouse over the checkmark).
If you find any interesting facts or have suggestions for how I could improve the tool, let me know in the co...
I just created a cool CF tool called [LGM
Sniper](https://wumbogames.com/CompetitiveProgramming
112.
C++20 is back In contests and in the custom invocation, after a pause, one can again find GNU G++20 64-bit. However, the [slowdown issue](https://codeforces.me/blog/entry/126654) because of which the language disappeared persists, there are still snippets of code that can slow down the execution on Codeforces servers by a factor of 100 or so.
So what is the official position of Codeforces headquarters and of our community on that?
1. Are there general methods of constructing testcases that can exploit this GCC/Windows bug and therefore slow down solutions (like there are [anti-hash tests](https://codeforces.me/blog/entry/62393) for solutions using `unordered_set`)? Are there methods that slow down both contest mode and custom invocation mode (since [submission:249807302] is only slow in the contest mode whereas [submission:253289473] is only slow in the custom invocation)?
2. If so, should we expect such tests in future contests, will such methods be used during the test preparation in co...
4. Is there no bug in 32-bit version? If it's 64-bit only, maybe CF team could
add a C++20 32-bit
113.
Простые оптимизации ДП Нет, я не имею в виду Convex Hull-оптимизацию и оптимизацию Кнута. Я имею в виду более простые и распространённые методы.
_Это продолжение моей серии постов о ДП, чтобы понимать обозначения, читайте [пролог](https://codeforces.me/blog/entry/102542)._
### Переходы из отрезка
Очень часто после построения графа динамики можно обнаружить, что переходы в то или иное состояние ведут не из случайного набора вершин, а из некоторого отрезка по одному из параметров. _Например, из состояний `dp[i][j]`, при некотором константном `i` и при `j`, лежащим в отрезке `[a, b]`._
**В таком случае можно не реализовывать каждый переход в отдельности, а реализовать все скопом.** Если под реализацией перехода понимается прибавление из `dp[A]` в `dp[B]`, то можно воспользоваться префиксной суммой. Если же нам нужно найти _наилучший_ переход в `dp[B]`, то можно воспользоваться деревом отрезков.
#### Пример
[AtCoder DP Contest: Задача M](https://atcoder.jp/contests/dp/tasks/dp_m) _Найти количе...
114.
Undesired -fpermissive behaviour of CF GCC compilers Apparently $24$ hours without sleep and months of coding in scripting languages had the effect on me... During the contest I submitted this [submission:310073998]
_Quick challenge: pause reading and find the bug in my solution_
[Link to original art](https://www.artstation.com/artwork/8elamx)
When I found out that I failed a div3D on not properly using `int64_t` I was quite surprised, because I double checked that I did it right. I felt very stupid at first, but then I got surprised even more when I figured how exactly this happened. Take a look at the line $25$
~~~~~
const& r = cr[i];
~~~~~
It appears that GCC allows to declare variable references omitting type declaration completely when cv-qualifier is present. Yes, without invoking type deduction using `auto`. To be precise expression in form
~~~~~
<cv-qualifier> &<variable-name> = <value>;
~~~~~
appears to be well-formed an...
Undesired -fpermissive behaviour of CF GCC compilers, It appears to be compilable on all $3$ available G++ compilers on CF and on my
local GCC `11.2.0
115.
August Classes & Huawei IOI Camp 2024 Hello, everybody.
<spoiler summary="August CP School, cancelled">
August CP School
----------------
I'm again organizing group classes after trying the format [last month](https://codeforces.me/blog/entry/129878). There will be problem-solving, homework, and recordings. I can make a lecture on some topic per requests. There will be two groups. You can attend both, just don't spoil anything in the lower group. (Btw. I also want to do 3-month weekly classes after Summer.)
1. Medium group: [16:00-17:30 CEST](https://www.timeanddate.com/worldclock/fixedtime.html?iso=20240808T16&p1=262&ah=1&am=30), 90-minute lessons, CF max rating 1500-1800
2. Hard group: [18:00-20:00 CEST](https://www.timeanddate.com/worldclock/fixedtime.html?iso=20240808T18&p1=262&ah=2), 2-hour lessons, CF max rating 1900-2200
- Dates: August 8 to 26 or 29 (6 meetings in total)
- Every Monday and Thursday
- Price: 200 USD per person (so around 20$ per hour, or 13$ if you're going to attend both groups)...
=20240808T16&p1=262&ah=1&am=30), 90-minute lessons, CF max rating 1500-1800 2.
Hard group: [18:00-20:00
116.
Codeforces Round #589 (Div. 2) Editorial Hello, I hope all of you enjoyed my contest!
[tutorial:1228A]
**[Behind story of A]**
- There is a successful hack for A. I am really surprised.
- My code: https://codeforces.me/contest/1228/submission/61578842
[tutorial:1228B]
**[Behind story of B]**
- There is no behind story.
- My code: https://codeforces.me/contest/1228/submission/61578841
[tutorial:1228C]
**[Behind story of C]**
- Initial version of C statement consists of tons of mathematical formula. CF team and testers requested me to reduce amount of mathematical formula.
- This problem was added before a week to the round. If there was no such C, the balance would be bad.
- Thanks for ~dorijanlendvaj,2019-09-29 , he improved test data for C a lot!
- My code: https://codeforces.me/contest/1228/submission/61578856
[tutorial:1228D]
**[Behind story of D]**
- Same as C, I wrote tons of mathematical formula in D. After CF team's request, I reduced the amount of formula.
- This is my personal...
- Same as C, I wrote tons of mathematical formula in D. After CF team's
request, I reduced the
Tutorial of Codeforces Round 589 (Div. 2)
117.
Group tutoring Hi. I offer classes for groups of 2-3 students.
I teach competitive programming with a focus on problem-solving.
There won't be many lectures because I can send you an article/video link instead.
The lesson cycle is usually: I choose a problem, you say your thoughts and ideas, I comment on incorrect ideas, and we talk about the valid solution(s), possibly with drawings and pseudocode.
In beginner groups, I might ask you to implement something, C++ or Python preferred.
There's a lot of homework and you're expected to practice a few hours per week.
We might spend half a lesson talking about 1-2 homework problems from last week.
This is intended.
We use Google Meet, shared whiteboard, and a collaborative editor Codebunk.
After a lesson, you get a video recording and a codebunk with code/text history like this one https://codebunk.com/pb/3501100331621/.
This allows you to copy links and code easily.
There's a Discord group chat to ask questions between classes.
- 1.5h les...
competitions, IOI gold medalist coach. At some point: CF legendary
grandmaster, TC target,CF top
118.
Improved Codeforces Extension : CF Analytics Hello delta hunters!!
I forked and added few features to the popular [CF Analytics Extension](https://github.com/ApoorvaRajBhadani/cf-analytics). You can review it here : [CF Analytics: Ashray's Fork](https://github.com/Ashrayyy/cf-analytics).
Below are the main features, that I personally really wanted, and are implemented.
- The extension can reproduce graph of problems solved overall, problems solved during contest and problems solved after contest separately.
###### Note: Problems solved during virtual contests will be considered Solved during contest.
<table>
<tr>
<td width="50%">
<img src="https://github.com/Ashrayyy/cf-analytics/assets/101005702/431e8aa4-0a56-45b5-a692-2eb0e57acf44" width="100%" />
</td>
<td width="50%">
<img src="https://github.com/Ashrayyy/cf-analytics/assets/101005702/302e5826-867e-48f3-b68c-f600c773e1bb" width="100%" />
</td>
</tr>
</table>
- Now, you can click on the graphs to generate a l...
119.
Introducing cp-notes.com — a place for your interesting competitive programming problems! **Update in 2025:** I've mostly forgotten about this site, but recently something rather funny came up: I was looking for a way to get a relatively large number of free subdomains (name of the url not mattering at all), when I realized I was still paying for cp-notes.com. So I tried to login to the AWS account owning the domain (which I haven't done in years), but it prompted me to check the email associated with it for a verification code... Unfortunately, I had used my Columbia university email out of convenience, but that has been disabled! (which I suspect has something to do with me dropping out?) I actually can still update the website (not that I was planning to) since I have some keypair saved which allows programmatic access, but not being able to use the console is a huge pain. I could also contact AWS support, but I feel like that would take a while...
Anyways, it is time for me to express some of my frustrations: Fuck Columbia University! In addition to disabling my emai...
Oh yeah, you can also link your CF account in your settings page, and have your
username color
120.
Lockout Bot for codeforces Hello everyone, I'm sure many of you have been watching and enjoying the recent [lockout streams](https://www.youtube.com/watch?v=bBNIIg8REUU&feature=youtu.be) organized by [user:ecnerwala,2020-06-08] and [user:scott_wu,2020-06-08] and would be wanting to compete with your friends in a similar format. I have made a discord bot that does exactly the same. The source code for the bot can be found [here](https://github.com/pseudocoder10/Lockout-Bot).
The bot gives a set of 5 problems with ratings from [x, x+400], where x is chosen by you and the point distribution is [100, 500]. The first person to solve a problem gets the points for it. The match ends when someone has >= 800 points or 45 minutes have been elapsed. The bot automatically updates the status when someone solves a problem but you can also do it manually.
To invite the bot to your discord server, click [here](https://discord.com/oauth2/authorize?client_id=669978762120790045&permissions=0&scope=bot). If you don't wanna ...
identify <cf username>`. The bot will give you a random string, which you have
to set as your first
121.
AtCoder Grand Contest 073 Announcement We will hold AtCoder Grand Contest 073.
This contest counts for GP30 scores.
- Contest URL: https://atcoder.jp/contests/agc073
- Start Time: http://www.timeanddate.com/worldclock/fixedtime.html?iso=20250928T2100&p1=248
- Duration: 180 minutes
- Number of Tasks: 4
- Writer: [user:maroonrk,2025-09-27]
- Tester: [user:maspy,2025-09-27], [user:HIR180,2025-09-27]
- Rated range: 2000 -
The point values will be 700-1000-1400-1700.
We are looking forward to your participation!
**UPD and announcement about the AI usage in this contest**
First of all, I'm terribly sorry for what happened today; Problem C was solved by an AI.
It is legal to use AI in AGC contests.
However, this rule was adopted under the assumption that we would not provide problems that are solvable solely by AI.
What makes the situation worse is that, as far as I know, only paid plans are able to solve this problem.
This is especially serious, and I fully accept your frustration.
Of course, I te...
I should have paid more attention to the development of AI. I knew that some
hardCF/IOI/ICPC/PE
122.
Codeforces Round #765. One of the contest authors is entertaining again. Hello, Codeforces! Codeforces #765 (Div. 2) took place not so long ago, and one of its authors was me. When an hour had passed since the contest started, I decided to remember the old days and start making fun of the cheaters.
To that end, I decided to take a look at the old cheater groups and see how they were doing.
The first group I looked into — is the group where the admin is actively fighting against cheaters. So, there really weren't any cheaters on the round. I think that's a success.
The second group I checked out — it's a group with 2.5k users. I decided that just going in there would be too boring, so I decided to write there that I was a contest author. Here's the reaction of the users:
<spoiler summary="Picture">
<a href="https://imgbb.com/"><img src="https://i.ibb.co/0Xt07qt/Screenshot-20220112-164100.png" alt="pic1" border="0"></a>
</spoiler>
Can you imagine?! I write that I am the author of the contest, and I am offered to leak the solution ...
But then a man wrote to me and asked me what my nickname on CF. I told him
honestly that I am
123.
CodeCraft-22 and Codeforces Round #795 (Div 2.) **Namaste Codeforces!**
We are excited to invite everyone to **CodeCraft-22** which will take place on [contest_time:1691]. It is rated for all people who are below **<font color="#ff8c00">yellow</font>**! (rating under 2100)
The [**Programming Club at IIIT-H**](https://iiit-programming-club.github.io/) organizes this event under Threads'22, as a part of our techno-cultural fest [**Felicity, IIIT Hyderabad**](http://felicity.iiit.ac.in).
<img src="/predownloaded/91/7b/917ba5fc64144c1effe15bd932cfb39dab2f7fda.png" width="400px">
There are **6** Problems with **2 hours** to solve them. The scoring distribution will be released soon!
We have worked hard to keep the statements clean and the pretests strong! After the contest, **step-wise tutorial** and **video tutorial** will be released along with the code!
- This round was wonderfully co-ordinated by [user:darkkcyan,2022-05-27] and [user:KAN,2022-05-27] orz!
- This round was a joint effort by [user:rahulgoel,2022-05-...
set to India in their CF profile will qualify as Indian participants., We will send a CF personal message to the winners of the T-shirts soon.
Announcement of CodeCraft-22 and Codeforces Round 795 (Div. 2)
124.
Can we upgrade contest divisions? Often these days it feels like the problems are too hard for their positions but it's almost never the other way. I'm not saying these problems are fundamentally bad — it's good to confront them to improve our problem solving skills — but it's not very healthy in a contest's perspective.
Let me bring up some examples:
**Example 1.** Yesterday's contest, [contest:1973], had no official all solves. Most of Grandmasters, and even LGMs suffered from the difficulty of problem F. Less than 1% of the official participants were able to solve any of the harder half of problems of the contest.
**Example 2.** [contest:1971], had the hardest problem [problem:1971H] that requires knowledge of 2-SAT. Only a very few official participants were able to solve it even though the topic was pretty well-known, just because the subject's level itself is too high. I think this is one of the harder topics even for the last problem of Div. 3, but it appeared on a Div. 4 contest.
**Examp...
solves often come from people whose rating is low just because they didn't
participate in manyCF
125.
After-stream thoughts Hello again.
Next stream will start tomorrow (Wednesday) at 2pm CEST ([your timezone](https://www.timeanddate.com/worldclock/fixedtime.html?msg=Errichto+Stream+%233&iso=20181003T14&p1=262)), solving problems from POI 22, round 1 ([link](https://szkopul.edu.pl/p/default/problemset_eng/oi/22)).
It will be on my [Youtube channel](https://www.youtube.com/channel/UCBr_Fu6q9iHYQCh13jmpbrg).
I did two streams this weekend that you can now find on my youtube [channel](https://www.youtube.com/channel/UCBr_Fu6q9iHYQCh13jmpbrg). The first with quite easy Codeforces problems that I solved from scratch (I haven't seen them before), also implementing them. The second one with a bit harder Topcoder problems that I've solved before, but I didn't necessarily remember the solution now — and for those I only described the solution, without implementation. The streams were fine but there are a lot of things to improve.
I wanted to see two things:
1. Can I stream? Am I able to try to sol...
Yet another possibility is popularizing CP. Solving 50 easiest problems on CF
or SPOJ wouldn't be
126.
DP Group Class in August 2025 _The blog publish date is incorrect, should be around 12.07.2025. CF saved the draft date._
Hi, I'm organizing paid group classes on Dynamic Programming in August. Last year, I did something similar with general problem-solving ([link](https://codeforces.me/blog/entry/129878)).
There will be two groups of different style and difficulty.
Each group gets 8 lessons of 1h30m each, from 28.07.2025 to 24-31.08.2025.
Group size up to 12 people.
You get access to recordings and problems from both groups, but you should actively attend only one.
There's a lot of homework, some to be discussed next lesson.
I will create new original CF/Polygon problems, especially for the easy group.
These problems will eventually be published for everybody!
**Price**: 250 EUR with a small country-based discount.
**Registration**: You should pay via [link (Stripe)](https://buy.stripe.com/7sY3cx2nD5Oh8CQ7w96wE03) and choose the group there. I will send you the Discord invite link via e-mail. Co...
contest) for CF+original problems. YouTube for (private) recordings of the
class., Reach out to me (CF or [email protected]) if you have any questions. Cheers., _The blog publish date is incorrect, should be around 12.07.2025. CF saved the
draft date._
127.
How not to read problem statements I wanted to write super-harsh blog but then I decided to split it into educational part and whining part. This is whining part. To read some cool stuff go [here](https://codeforces.me/blog/entry/62730). This one will also be educational on a slightly different topic. And there will be A LOT of harsh stuff.
I'm not saying that I'm perfect. That round had some really bad problemsetting issues. Problem E was googleable, problem D had very weak tests against very stupid solution (so stupid that none of the authors and testers come up with it, moreover, it is so counter-intuitive that I don't know how so many people independently did it). In two constructive problems participants could look at jury's output, the feature of which I didn't know. And I'm sorry for these issues. BUT. [This comment](https://codeforces.me/blog/entry/62614#comment-466588) has very negative rating despite being so true. And no one are here to back me up. So I want to speak my truth as I always do.
Let me ac...
setting rounds on CF. That looks like a good thing for all of us. You don't
want to solve my problems, I also think that CF should abandon the practice of answering every
clarification if possible. It, [Problem C from 2nd division of CF Round
#518](https://codeforces.me/contest/1068/problem/C) gets
128.
Some IOI 2026 Statistics Hey everyone, IOI2026 is over! Congratulations to everyone who participated for their hard efforts. While all the contestants were struggling to solve the problems, I had some free time so I looked into some stats.
Some notes before we get into things:
- It is entirely possible that some of these are wrong/incomplete/misleading. I only made these to satisfy my own curiosity, and they are not an authoritative source
- I expect this to be particularly true for medal cutoffs, where I have not accounted for unofficial contestants
Thanks to all the organizers for making IOI2026 a great event!
[cut]
I try to break them down by category so that this blog doesn’t get too long:
IOI vs Codeforces
------------------
This has been well researched before, ([here](https://codeforces.me/blog/entry/145502), [here](https://codeforces.me/blog/entry/134654), [here](https://codeforces.me/blog/entry/68981), [here](https://codeforces.me/blog/entry/57700), and [here](https://codefor...
### Are some problems more CF-like than others?
129.
A simple introduction to "Segment tree beats" **This blog is just a back up of [http://codeforces.me/blog/entry/57319](http://codeforces.me/blog/entry/57319) to prevent this tutorial from discarding after [user:jiry_2,2018-10-17] is banned.**
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 algorit...
are quite new for competitors in CF although they are well known in China from
the final standings
130.
Codeforces command-line interface tool Codeforces Tool is a command-line interface tool for Codeforces.
It's written in golang (without any browser driver). And it's fast, small(only about 7 MB), cross-platform(Windows, macOS, Linux) and powerful.
You can find the source and binary files in my git repo <https://github.com/xalanq/cf-tool>
You can download the tool from release page <https://github.com/xalanq/cf-tool/releases>
Features:
* Support Contests, Gym, Groups and acmsguru.
* Support all programming languages in Codeforces.
* Submit codes.
* Watch submissions' status dynamically.
* Fetch problems' samples.
* Compile and test locally.
* Clone all codes of someone.
* Generate codes from the specified template (including timestamp, author, etc.)
* List problems' stats of one contest.
* Use default web browser to open problems' pages, standings' page, etc.
* Setup a network proxy. Setup a mirror host.
* Colorful CLI.
 that is scheduled to happen on **18-26 February 2023** in Osijek, Croatia. The camp is hosted by the [Department of Mathematics at J. J. Strossmayer University of Osijek](https://www.mathos.unios.hr/index.php/en/) and is brought to you by the organizing team of [user:adamant,2022-12-21] (that's me!), [user:-is-this-fft-,2023-01-06], [user:mblazev,2023-01-06], [user:Tomx,2023-01-06] and [user:antontrygubO_o,2023-01-05].
The camp is inspired by various competitive programming camps that we attended during our active years in ICPC, and is aimed to help college students prepare for ICPC regional contests and finals. The camp will consist of 7 ICPC-style contests and 2 days off.
Up until now, most top competitive programming training...
- [user:MiracleFaFa,2023-01-06] — a guy who changed CF username every year and
labeled the
132.
"Teams" going to the 2021 Facebook Hacker Cup Finals, with tourist factors Good Morning! I'm no [user:Laggy,2021-11-24], but I thought I would continue the tradition of making a "Teams" list for the 2021 Facebook Hacker Cup Finals, since I always found the ICPC World Finals lists fun. So here goes!
Facebook Hacker Cup 2021 Finalists
==================
<table>
<thead>
<tr>
<th>Rank</th>
<th>Handle</th>
<th>Country/Region</tb>
<th>Codeforces Rating</th>
<th>Hacker Cup R3 Place</th>
<th>[user:tourist,2021-11-24] factor*</th>
</tr>
</thead>
<tr>
<td>1</td>
<td>[user:tourist,2021-12-06]</td>
<td>Belarus</td>
<td>3870</td>
<td>1</td>
<td>50.00%</td>
</tr>
<tr>
<td>2</td>
<td>[user:Retired_MiFaFaOvO,2021-12-06]</td>
<td>China</td>
<td>3681</td>
<td>13</td>
<td>25.20%</td>
</tr>
<tr>
<td>3</td>
<td>[user:Radewoosh,2021-12-06]</td>
<td>Poland</td>
<td>3418</td>
<td>2</td>
<td>6.90%</td>
</tr>
<tr>
<td>4</td>
<td>[user:Petr,2021-12-06]</td>
<td>Switzerland</td>
<td>3...
Does CF Rating Predict Hacker Cup Performance? ================== Here's a
graph of the finalists
133.
Team dashboard Hey Codeforces!
I made a thing and I want to share it with the community.
We wanted to keep track of achievements and submissions in our team and using Codeforces status page is not really handy. Therefore, I introduce to you **Team dashboard**.
Here you can find some kind of a timeline for each member of your team, showing last submissions, contests and even blog posts on Codeforces.
Using this script is pretty easy, just specify up to 3 handles in the address, separated by semicolons (please check below). Also please keep in mind that this script was made in 3-4 hours, so there is almost certainly a handful of bugs. Also, the script asks a lot of queries to the Codeforces API, so sometimes not all requests are answered. You should wait a couple of seconds and try again.
Visit right now and bookmark your team:
http://weaselcrow.com/pro/cf/team/?h=Radewoosh;Errichto;kostka
You can also compare yourself with you...
, http://weaselcrow.com/pro/cf/team/?h=Radewoosh;Errichto;kostka, http://weaselcrow.com/pro/cf/team/?h=kostka;olekluka, http://weaselcrow.com/pro/cf/team/?h=mnbvmar&d=60
134.
Congratulations! Congratulations to Chinese Team([user:Rebelz,2023-09-01], [user:cnnfls_csy,2023-09-01], [user:Laurie,2023-09-01] and [user:He_Ren,2023-09-01]), USA Team([user:rainboy,2023-09-01], [user:PurpleCrayon,2023-09-01], [user:GusterGoose27,2023-09-01] and [user:EmeraldBlock,2023-09-01]), Russian Team([user:bashkort,2023-09-01], [user:valerikk,2023-09-01], [user:stepanov.aa,2023-09-01] and [user:Qwerty1232,2023-09-01]) and Japan Team([user:blackyuki,2023-09-01], [user:penguinman,2023-09-03], [user:PCTprobability,2023-09-03] and [user:Kodaman,2023-09-03]) for getting 4 golds for their countries!
Also congrats to [user:blackyuki,2023-09-01] for submission at the very last second gaining +29 points, and to [user:He_Ren,2023-09-01] for a fabulous comeback on Day 2.
This year's contest is full of excitement, not only for the contestants but also for spectators like me watching the Top 2 battling for points on D2C, Yuki's 'extraordinary second half', and all other contestants' hard work. Congra...
(Sorry but I cannot find the CF accounts for all Japanese Team members, and
also I'm assuming there
135.
The relationship between IQ and CF Rating Hello Codeforces,
I hope everyone is having a great day.
Recently I have been seeing many articles and journals on whether certain skills are inherent or acquired. These articles talk about whether, for a certain skill, one is simply born good at it, or someone works to develop this skill, and over time builds it.
According to R. Plomin, (https://www.scientificamerican.com/article/is-intelligence-hereditary/), genetic differences account for a shocking 50% of cognitive disparity among people. The rest is affected by factors like environment and pure hard work.
Does this mean that those who draw a good hand of cards are more likely to be successful?
Let’s get more specific.
In a study done by the University of Washington, (https://www.sciencedaily.com/releases/2020/03/200302103735.htm), participants “took eight different tests: one that specifically covered numeracy; one that measured language aptitude; and others that assessed attention, problem-solving and memory.”...
The relationship between IQ and CF Rating, would like to compare participants' IQ with their CF rating and see if there
is a direct correlation
136.
CF Country Standings — Fresh and Updated [](https://i.ibb.co/93gCGgc8/Screenshot-2026-03-01-040626.png)<br/>
_click to enlarge the image_
Now you can add a **“Country Standings”** tab to Codeforces **contest/gym standings** pages.
The goal is simple: quickly see how participants from a specific country performed — including **teams** — without manually searching the full standings!
#### Key features
- **New “Country Standings” tab**: Appears next to the normal standings menu and uses URL mode `?cs=country` to make it ready to share with friends!
- **Country filter for both individuals & teams**: Works for solo participants and teams automatically, A team is included if **any member** of the team matches the selected country
- **Extra profile info**: Shows **avatar**, **rating pill**, **full name + city**, and **organization/institute** (if present on CF profile)
- **Uses Codefo...
CF Country Standings — Fresh and Updated, ####2. Install the **CF Country Standings** script from
[here](https://greasyfork.org/en/scripts
137.
True Codeforces Heatmap (Based on Rating) — Chrome Extension # Rating-Based Heatmap: Visualize Real Progress!
**[Download from Chrome Web Store](https://chromewebstore.google.com/detail/codeforces-rating-based-h/heajdhmohlobjebkgkpdomkaihaghkgb)**
Are you tired of the default Codeforces heatmap that merely tracks the number of problems solved without reflecting the actual effort behind each? Wouldn't it be great to see how much hard work you invested each day? Introducing the **Codeforces Rating-Based Heatmap Extension** — a clearer way to reflect your real progress!
---
## Why Settle for Less?
The default Codeforces heatmap colors days based solely on the quantity of problems solved. It doesn't differentiate between solving easy Div3 problems or tackling complex Div1 challenges. This approach doesn't truly represent the difficulty you faced.
Additionally, some users may focus on solving easy problems just to maintain a streak. While streaks can be motivating, they don't always reflect real progress in problem-solving. True p...
](https://addons.mozilla.org/en-US/firefox/addon/cf-rating-based-heatmap/)
138.
fix problem ratings pls (the rainboy effect) [The first few paragraphs of this blog, above the first spoiler, are just for story purposes and are irrelevant to the problem statement]
One fateful day, a naive and young version of me from about 5 days ago had a dream. A dream to stop being so terrified of hard problems. A dream to conquer the most challenging stuff that CF ever had to offer. A dream to... ok, enough of that, that's so stupid. Basically, I've been grinding 3500s lately, mostly for fun (and my god, has it been fun).
Two days prior, I found myself solving [1919G](https://codeforces.me/contest/1919/problem/G). That went fine, but after getting it, I noticed that despite G being 3500, it wasn't even the hardest problem in the contest — that honor went to the next problem, [1919H](https://codeforces.me/contest/1919/problem/H) (the subject of this blog), which had **no solves in-contest**. I concluded that I would have to be insane to try that, and moved on to other problems.
Well, yesterday, I decided I...
being so terrified of hard problems. A dream to conquer the most challenging
stuff thatCF ever had
139.
Codeforces achievements Hi!
I'd like to introduce my project to Codeforces community. My project's name is 'Codeforces achievements' and as you can probably guess it is about achievements! Your achievements on Codeforces site. For those who do not like my long posts I should mention that the link to the site is at the end of this post.
Introduction
==================
Last summer I left my last job, so I have quite a lot of spare time which I could invest in my hobby. Also I decided to learn something more or less new for me (for last several years my job was to write desktop software for Windows), so I decided to put my hands on some modern popular web-framework. At that point I chosen Python and Django, cause it was matching one position I applied. So I had a time and I wanted to learn Django. The best way to learn something is to practice in it, so I started looking for the idea for the site to create (didn't want to write yet another forum engine). Luckily at that point I ran into [user:I_love_...
Finally the link: [CF-achievements](http://cfa.yuldashev.net/)
140.
Educational Codeforces Round 63 Editorial [problem:1155A]
<spoiler summary="Tutorial">
[tutorial:1155A]
</spoiler>
<spoiler summary="Solution (Vovuh)">
~~~~~
#include <bits/stdc++.h>
using namespace std;
int main() {
#ifdef _DEBUG
freopen("input.txt", "r", stdin);
// freopen("output.txt", "w", stdout);
#endif
int n;
string s;
cin >> n >> s;
for (int i = 1; i < int(s.size()); ++i) {
if (s[i] < s[i - 1]) {
cout << "YES" << endl;
cout << i << " " << i + 1 << endl;
return 0;
}
}
cout << "NO" << endl;
return 0;
}
~~~~~
</spoiler>
[problem:1155B]
<spoiler summary="Tutorial">
[tutorial:1155B]
</spoiler>
<spoiler summary="Solution (Roms)">
~~~~~
#include<bits/stdc++.h>
using namespace std;
int n;
string s;
int main(){
cin >> n >> s;
int cnt1 = (n - 11) / 2;
int cnt2 = cnt1;
string res = "";
for(int i = 0; i < n; ++i){
if(s[i] == '8'){
if(cnt1 > 0) --cnt1;
else res += s[i];
...
fore(cj, j, sz(mat[i])) mat[i][cj] = norm(mat[i][cj] - mul(cf, mat[j][cj])); } }, swap(mat[j], mat[nid]); fore(i, 0, sz(mat)) { if(i == j) continue; int cf =
mul(mat[i][j
141.
Virtual participation is great! I want to promote the option of virtual participation in contests, since I noticed it's not used as often as it could be. Perhaps it's just not something people keep in mind, but I'm sure that if you use it often, it will help you a lot.
What does it do? It allows you to compete in past contests as if you were actually competing when the contest took place — there are only two differences:
- it's unrated
- there are no hacks and so far, no pretests
Virtual participation is a great way to compete without worrying about your rating. Instead of making a fake account that you then throw away, you can just participate with your real account with no risk. If you care about embarassing yourself with poor performance, don't worry about it. Nobody cares.
Virtual participation is also a great way to train. Unlike regular upsolving, you have a timer, so you can't just take your time trying stupid shit or overthinking. This way, it's much better for training fast thinking. Takin...
correspond to normal CF rounds. During the contest, the scoreboard you're
shown is ACM-style, but it's
142.
Experimental Educational Round: VolBIT Formulas Blitz Editorial #### [A. Again Twenty Five!](http://codeforces.me/problemset/problem/630/A)
The problem of getting the last two digits is equivalent to the problem of getting the number modulo $100$. So we need to calculate $5^{n} \mod 100$. According to
[cut]
the rules of modular arithmetic
$(a \cdot b) \mod c = ( (a \mod c) \cdot (b \mod c) ) \mod c$
So
$5^{n} \mod 100 = ((5^{n-1} \mod 100) \cdot 5) \mod 100$
Let's note that $5^2=25$. Then
$5^3 \mod 100 = ((5^2 \mod 100) \cdot 5) \mod 100 = (25 \cdot 5) \mod 100 = 25$
$5^4 \mod 100 = ((5^3 \mod 100) \cdot 5) \mod 100 = (25 \cdot 5) \mod 100 = 25$
And so on. All $5^n \mod 100$ are equal to $25$ for all $n \ge 2$.
So to solve the problem one need to just output `25`. There is no need to read $n$.
#### [B. Moore's Law](http://codeforces.me/problemset/problem/630/B)
Every second the number is multiplied by $1.000000011$. Multiplication several times to the same number is equivalent to exponentiation. So the formula i...
- $[0, 89]$ degrees — $0$ turns, - $[90, 179]$ degrees — $1$ turn, - $[180, 269
]$ degrees — $2, - $[0, 89]$ градусов — $0$ поворотов, - $[90, 179]$ градусов — $1$ поворот, -
$[180,269
143.
[Tutorial] Fully Dynamic Trees Supporting Path/Subtree Aggregates and Lazy Path/Subtree Updates Hey everyone! Usually when I post stuff on CF I just drop an external link to my blog, but this time I would like to also paste the same content natively on CF to see if it leads to more engagement. You can read the exact same article on my blog [here](https://mzhang2021.github.io/cp-blog/ds5/).
Also, special thanks to [user:alien_lover,2022-06-10] and [user:gabrielwu,2022-06-10] for proofreading.
---
## Motivation
We will solve **[this problem](https://dmoj.ca/problem/ds5)**.
## Prologue
The original name of this problem is [SONE1](https://darkbzoj.cc/problem/3153), originating from the fiery depths of Chinese OI. Amazingly, the Chinese have figured out how to augment the link/cut tree to solve this problem (ref [1](https://www.cnblogs.com/clrs97/p/4403244.html), [2](https://blog.csdn.net/iamzky/article/details/43494481)). Maintaining subtree aggregates with a LCT isn't difficult, and there's a well-known [Codeforces article](https://codeforces.me/blog/entry/67637) ...
Hey everyone! Usually when I post stuff on CF I just drop an external link to
my blog, but this
144.
The relationship between IQ and CF Rating (continued) Hello Codeforces,
Sorry for the delay on the update blog.
This is a continuation of my blog on the relationship between IQ and CF Rating. [Previous Blog Post]
(https://codeforces.me/blog/entry/90996)
First off, I would like to begin by expressing my gratitude to everyone who submitted the form. In the end, I received over 140 submissions, which is a crazy number. Thank you to everyone.
After putting all the data into excel, I saw there was almost no correlation in any of the categories.
Here are the scatter plots for all the categories:
Overall:
The R^2 value here is very low, and although you can see a general increase in rating as IQ increases, it is slight and full of outliers.
Memory IQ:
The Memory IQ section also had a low R^2 value, and although rating once again increases slightly with IQ, there are man...
The relationship between IQ and CF Rating (continued), This is a continuation of my blog on the relationship between IQ and CF Rating.
[Previous Blog Post
145.
Invitation to TOKI Open 2018 Hello!
After we successfully organized [TOKI Open 2014](http://codeforces.me/blog/entry/12286) and [TOKI Open 2017](http://codeforces.me/blog/entry/51512), we are going to do the similar thing this year. We will open our current IOI Selection Contest to international participants. We call it **TOKI Open 2018**. TOKI stands for Tim Olimpiade Komputer Indonesia, or Indonesian Computing Olympiad Team in English.
Objectives of us opening this contest include, but not limited to:
- Measure the performance of our IOI trainee compared to other countries' IOI trainee.
- Foster friendship between Indonesia and other IOI participating countries.
- Test Indonesia's scientific committee capability in setting IOI-level problems.
- Test Indonesia's technical committee capability in hosting IOI-level contests.
The contest will have very similar format with IOI, as this contest will be used to select our Top 4 and train our Top 4 for IOI. We expect that the difficulty of the contest i...
://competition.ia-toki.org/contests/269). The problems of TOKI Open 2017 are
available for upsolving in
146.
CF community & CF API I'd like to start a discussion on how CF community is involved or at least notified about the changes happening in CF and in CF API in particular.
It seems that there are people more or less constantly working on implementing some new features on CF. And sometimes we can see blog posts from [user:MikeMirzayanov,2015-05-28] or somebody from CF team highlighting a list of changes which happened during some period, but these posts are very rare, they are published like once or twice per year, while it seems that we have something changed almost every month. CF has very strong community, some people from the community do something on their own built on top of CF web site or CF API ([submit solution from command line](http://codeforces.me/blog/entry/14786), [VIM plugin](http://codeforces.me/blog/entry/16635), my CF achievements project). For all of us it is critical to be aware of changes happening on CF. For example this submission script stopped working after CSRF security fix. Ano...
CF community & CF API, the development of CF API. Obviously CF API is implemented for the community
so we should have a way, 2. [Ivan's introduction of CF API blog
post](http://codeforces.me/blog/entry/12520)? But it's, CF. And sometimes we can see blog posts from [user:MikeMirzayanov,2015-05-28]
or somebody fromCF, I'd like to start a discussion on how CF community is involved or at least
notified about the
147.
Anti-hash test. Hi everyone! I wan't to share some interesting fact about how we're going to challenge almost every solution, that uses polynomial hashes modulo 2^64. We'll hack any solution, regardless of it's base (!), the only thing we need, that it's using int64 with overflows --- like many coders write hashing.
Keywords: Only today, only for you, ladies and gentlemen: we're gonna challenge [user:Petr,2012-07-19]'s solution in problem [problem:7D] from Codeforces Beta Round #7!
Is it interesting? Welcome reading after the cut.
[cut]
Firstly, for the most impatient of you. Here's the source of the generator:
~~~~~
const int Q = 11;
const int N = 1 << Q;
char S[N];
for (int i = 0; i < N; i++)
S[i] = 'A' + __builtin_popcount(i) % 2;
// this function in g++ returns
// number of ones in binary representation of number i
~~~~~
Let's try solutions of two CFBR #7 winners on this test: Petr Mitrichev's and Vlad Epifanov's.
[user:vepifanov,2012-07-18] solution doe...
. [user:burunduk1,2012-07-19] offered to post it on CF, so here it is :-)
148.
The Rating table History versions: [here](https://www.luogu.com/paste/46w0g5p4), link from Luogu. If this project is successful I'll make a similar table for Luogu.
[This](/blog/entry/126802) is someone else's chart. Due to the exclusion of interfering factors, the persuasiveness is stronger. (But I will still insist on making this less convincing version)
The columns: name = name, min = min, max = max, total = there're how many people in front of this line, current = there're how many people with you in this zone, proportion = proportion. Guess where you are?
<spoiler summary="The AtCoder Table">
</spoiler>
I made this thing purely out of boredom, and it would be my honor if it could be helpful to you.
I hope I will remember to update it every month.
<spoiler summary="Fun Facts:">
- Due to the stra...
149.
Educational Codeforces Round 49 Editorial [problem:1027A]
<spoiler summary="Tutorial">
[tutorial:1027A]
</spoiler>
<spoiler summary="Solution (PikMike)">
~~~~~
#include <bits/stdc++.h>
#define forn(i, n) for (int i = 0; i < int(n); i++)
using namespace std;
int main() {
int T;
cin >> T;
int n;
string s;
forn(_, T){
cin >> n >> s;
bool ok = true;
forn(i, n){
int k = abs(s[i] - s[n - i - 1]);
if (k > 2 || k % 2 == 1){
ok = false;
break;
}
}
cout << (ok ? "YES" : "NO") << endl;
}
return 0;
}
~~~~~
</spoiler>
[problem:1027B]
<spoiler summary="Tutorial">
[tutorial:1027B]
</spoiler>
<spoiler summary="Solution (Vovuh)">
~~~~~
import sys
lst = sys.stdin.readlines()
n, q = map(int, lst[0].split())
for i in range(q):
x, y = map(int, lst[i + 1].split())
cnt = (x - 1) * n + y
ans = (cnt + 1) // 2
if ((x + y) % 2...
li phi = dv; fore(i, 0, sz(fm)) { if(pw[i] > 0) { cf = merge
150.
[Tutorial] The command line: how to read input from file without #ifdef and much more #### Introduction
Instead of algorithms and math, I'll be doing something completely different here.
There is something that has bothered me for a long time: people not understanding
the tools they use to participate in contests and thus failing to take real advantage
of them. And it all goes back to not understanding the command line.
Some of you might say, "well surely, the command line is something obsolete and you're
only clinging to it because you [insert ridiculous amateur-psychology here]?"
No! The command line is not obsolete, and unless competitive programming becomes
so mainstream that software vendors start writing professional-grade tools aimed
specifically at us, it won't be.
[cut]
$~$
What is the command line? Any of these:
As you can see, it comes in many background colors and fonts, but the basic idea
is always the same. It's a box you can type commands in, which it will ...
. Many people copy-paste their code into the box on CF simply because amid all
this complexity
151.
Teams going to ACM-ICPC World Finals 2019 Completed List! [user:Luqman,2019-01-30] said he [isn't creating a chart](https://codeforces.me/blog/entry/64899?#comment-488294) of teams going to ACM-ICPC this year as he isn't competing, so I added in all the teams. Let's complete the table together!
For the teams where I don't know the cf handles of the team members, I put their names instead.
Europe
======
| Count | Country | Region | University | Member 1 | Member 2 | Member 3 |
| ----- | ------- | ------ | ---------- | -------- | -------- | -------- |
| 1 | Romania | SEERC | Alexandru Ioan Cuza University of Iași | [user:Juve45,2019-01-31] | [user:Cristian1997,2019-01-31] | [user:Sin,2019-01-31] |
| 2 | Switzerland | SWERC | ETH Zürich | [user:TimonKnigge,2019-03-08] | [user:zaspagety,2019-03-08] | Stefanie Zbinden |
| 3 | Netherlands | NWERC | Leiden University | [user:pimsp,2019-03-08] | Onno Berrevoets | Daan van Gent |
| 4 | Ukraine | SEERC | National Technical University "Kharkiv Polytechnic Institute" | [user:vlad8,2019-02-0...
For the teams where I don't know the cf handles of the team members, I put
their names instead.
152.
Codeforces Round #699 (Div. 2) Editorial [tutorial:1481A]
<spoiler summary="code">
~~~~~
#include <bits/stdc++.h>
using namespace std;
int main(){
int t;cin>>t;
while(t--){
int x,y;cin>>x>>y;
string s;cin>>s;
int u=0,d=0,l=0,r=0;
for(int i=0;i<s.length();i++){
if(s[i]=='U')u++;
else if(s[i]=='R')r++;
else if(s[i]=='D')d++;
else l++;
}
if(x > 0 && r >= x )x = 0;
if(x < 0 && l >= -x )x = 0;
if(y > 0 && u >= y )y = 0;
if(y < 0 && d >= -y )y = 0;
cout<<((!x && !y)?"YES":"NO")<<endl;
}
}
~~~~~
</spoiler>
Author: [user:Salem_Alwarawreh,2021-02-05]
[tutorial:1481B]
<spoiler summary="code">
~~~~~
#include <bits/stdc++.h>
using namespace std;
#define all(x) (x).begin(), (x).end()
#define fast ios::sync_with_stdio(false);cin.tie(0);
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
mt19937 rng(chrono::steady_clock::now()...
for(int i=n-1;i>=0;i--){ dp[i] = dp[i+1]; cf[a[i]]++; if(i ==
at[a[i]].first)dp[i] = max
Tutorial of Codeforces Round 699 (Div. 2)
153.
Problems that I authored so far Hi everyone!
Today I saw a discussion in AC Discord server about how many problems some people made for different competitions. It was sparkled by [this](https://codeforces.me/blog/entry/108595) CF entry. I haven't keep track of this before, so it made me curious to go through everything that I made and publish in a comprehensive list. I'm doing it mostly out of curiosity, but maybe it might be interesting for someone else too :)
[cut]<br>
| # | Date | Problem | Contest | Comment |
|-|-|-|-|
| 1 | Jan 2016 | [Sasha and Swaps](https://www.hackerrank.com/contests/infinitum14/challenges/sasha-and-swaps/problem) | Ad Infinitum 14 | Find a $T$-th root of a given permutation, while minimizing the number of swaps in which the root may be decomposed. |
| 2 | Aug 2015 | [Sasha and swag strings](https://acm.timus.ru/problem.aspx?space=1&num=1799) | Ptz Summer 2015. MIPT Contest | Compute the total number of distinct substrings on all edges of a given string's suffix tree. |
| 3 | ...
UPD: Added OKC 3 and CF Round #869.
154.
VK Cup 2017 Round 1 and CF Round 405 div1+div2 Hello.
The round 1 of VK Cup 2017 will take place on March 18 at 18:35 MSK (check your timezone [here](https://www.timeanddate.com/worldclock/fixedtime.html?day=18&month=3&year=2017&hour=18&min=35&sec=0&p1=166)), along with standard div1+2 Codeforces Round \#405.
The contest "VK Cup 2017 — Round 1" is for teams qualified from two Qualification Rounds.
The top 400 teams will advance to the Round 2, while other ones will have one more chance in the Wild Card Round 1 in April.
Those who don't participate in the VK Cup can take part in the Codeforces Round \#405 individually (problems will be available in English too).
All three rounds last 2 hours, and all are rated.
I want to thank: [user:KAN,2017-03-17] for his help in the contest preparation, [user:MikeMirzayanov,2017-03-17] that we are here, [user:AlexFetisov,2017-03-17] for testing, the VK company for this nice annual contest.
I'm a setter and I hope (and expect?) that you will enjoy the problems.
During the conte...
VK Cup 2017 Round 1 and CF Round 405 div1+div2, VK Cup 2017 Раунд 1 и CF Round 405 div1+div2
Announcement of VK Cup 2017 - Round 1
155.
RSHF — a Rating and Report System (Beta) # TL-DR
##### https://rshf.net/ (not adapted for mobile)
##### https://chromewebstore.google.com/detail/rshf/jmpohmlhicdbghkhageaiefhenmkdfcg
(the most recent version will be available at https://github.com/welcome-to-the-sunny-side/rshf-extension)
<br>
# TS-DR
<img src="https://i.ibb.co/NdgvgPLG/logo.png" alt="RSHF logo" style="float:right;width:300px !important;height:auto;margin-left:30px; margin-bottom:6px;">
Simply put, RSHF offers the ability for users to form groups that have their own self-contained rating, report and moderation systems. Deeper visual integration with the Codeforces website is offered through a browser extension. More details on each of these components follow:
### Groups
Group members are partitioned into three tiers:
1. Admins
2. Moderators
3. Members
Their roles and responsibilities closely match what you might think they do: Admins and Moderators process reports and take appropriate actions in response. Admins have additi...
I find cf ratings and subsequently username colors really helpful for mentally
filtering out
156.
A transient purple's message to Mike (About Official Participation) TL;DR: Participants who registered while yellow but who dropped to purple must be official participants.
Dear CF community,
Today was supposed to be a great day. After resting from an hour-long leisurely walk around the city, I went to Codeforces to compete in [contest:1934]. Problems A and B went smoothly, as expected. Then, an "Accepted" submission problem C followed suit. As I glanced at the right side panel of the "Problems" page, the minuscule number of solves accompanied by a two-digit count of C-solvers pushed me to continue this winning streak. To my surprise, I got D1. Then D2.
I was convinced that it would be a good day. After a brief saunter for snacks, I returned to my desk and checked the standings, expecting a top 50 finish, which would be my new personal best for a Div. 2 round.
Ranks 1 to 20 loaded onto my screen. "Hmm, was I in the top 20?" I questioned. Nope, not in the first ten nor the first fifteen. My username was nowhere to be found in the official st...
Dear CF community,
157.
[CF Extension] How Not To Feel Bad About Yourself I've made some tampermonkey extensions, I will make it available in Google Chrome Extension store if I'm not busy and lazy.
The extensions are for those whose souls are as weak as mine. ⸜(｡˃ ᵕ ˂ )⸝♡
## Simple UI
The first one is the simple UI if you want to just read or print CF problem statements in compact mode.
<spoiler summary="Screenshots">
</spoiler>
What it does:
- Remove the problem letters (to avoid bias)
- Make it clean and printable
- Auto display spoilers (for printing's sake)
<spoiler summary="CF Simple UI">
~~~~~
// ==UserScript==
// @name CF Simple UI
// @version 1.0
// @author hocky
// @match https://codeforces.me/problemset/problem/*
// @match https://codeforces.me/blog/entry/*
// @match https://codeforces.me/contest/*/problem/*
// @icon https://www.google.com/s2/f...
[CF Extension] How Not To Feel Bad About Yourself, ~~~~~ // ==UserScript== // @name CF Anti Ratist, ~~~~~ // ==UserScript== // @name CF Simple UI
158.
"YAY I'M GM!!!" blog So recently I became red on CF and my happiness knows no boundaries. For achieving the red color, I would like to thank those who helped me the most.
[user:dmikhalin,2024-12-01], my first informatics teacher who inspired me to do competitive programming
[user:gmusya,2024-12-01], teacher in B parallel of the main programming club in Russia, who answered many of my questions and helped my a lot
And all the teachers of parallel A!
I'd like to tell a timeline of my progression. Three years ago when I ended 8th grade I got a giant boost of motivation after I learned segment tree and dsu from CF EDU (that's actually useful try it). In summer school I got to expert (that was so cool, I still remember how I solved problem F from div 3, back then it was difficult). Then I continued to grind CF. Before 9th class I learned that there is a strong programming club from Tinkoff, so I decided to try to get there. Easily the best decision, this club gave me a gigantic boost during these ...
So recently I became red on CF and my happiness knows no boundaries. For
achieving the red color, I
159.
How to Solve Questions [Dominater Version] Recently, I got a request asking me to write down my thought process while solving questions. So, here is the promised blog.
I would like to thank [user:Iceknight1093,2024-08-30], [user:qwexd,2024-08-30], [user:Sana,2024-08-30], [user:Everule,2024-08-30] and [user:NovusStellachan,2024-08-30] for proof reading and suggesting edits in the blog. Special thanks to [user:satyam343,2024-08-30] for discussing most of the blog with me.
<h3> 1. Overview </h3>
The blog contains my solutions to $7$ problems in a wide range of ratings, starting from $1200$ all the way upto $2700$. Each problem has a step-by-step solution and you can notice how there are no large jumps in logic, but everything comes naturally. I do not claim that this is always possible in each problem, however I solve majority of CF problems in such a manner.
There are certainly other high rated people who will have completely different methods of solving. However, this is about what works for me. There are some meta ...
each problem, however I solve majority of CF problems in such a manner.
160.
A look at Competitive Programming post-hibernation Hello friends!
As I've recollected in a previous [post](https://codeforces.me/blog/entry/97566), I am an old competitor who hadn't really participated since ~2014, and recently got a bout of nostalgia to return to Competitive Programming. So, I did a couple Topcoder SRMs, suffered through some SNWS rounds, participated in [three regional 5hr competitions](https://contest.yandex.ru/3QF2021) on three consecutive days, and dozed off at every Codeforces contest I tried to wake up for.
A lot of things are still the same as 8 years ago: [user:tourist,2022-01-26] is still at the top, grey coders still ask for how many minutes to solve a problem before reading the editorial, Russian university teams [continue winning](https://icpc.global/worldfinals/results) ACM ICPC, and Snarknews never gives up on his alternate competition formats. But in this post, I want to focus on the new patterns that emerged since my last time around.
#### #1: Codeforces rounds timing
Did you know that t...
More generally, I can see there is a whole
[catalog](https://codeforces.me/catalog) here onCF
161.
A mystery of CF contribution distribution Recently, I'm interested in CF rating and contribution distribution, and I searched this.<br />
I already searched about mysteries of rating distribution ([Blog](http://codeforces.me/blog/entry/52470)), so I wrote here about contribution distribution. (Thank you for comments)<br />
<br />
**Method of Searching**<br />
<ul>
<li>[Here](http://codeforces.me/top-contributed) is the leaderboard of top contributor.</li>
<li>I searched the rank of contribution -51, -46, -41, -36,..., -1, 4, 9, 14,..., 164, 169, 174, 179. I searched the number of contributors too.</li>
<li>Then, I make the distribution graph.</li>
<li>Each range of the graph is [-inf, -50), [-50, -45), [-45, -40),... , [175, 180), [180, inf]. Each value of x-axis of the graph is the **minimum value** in range. For example, if the value of axis is 15, this means the range is [15, 20).</li>
<li>The minumum value of range [-inf, -50) is _inf_, but in distribution graph this value is _-1000_.</li>
</ul>
<br />
Howeve...
A mystery of CF contribution distribution, Recently, I'm interested in CF rating and contribution distribution, and I
searched this.
I
162.
Codeforces Data Scraping Websites There are many useful websites that extract data from Codeforces and give some statistics, problem recommendations, graphs, etc.
I thought that collecting them all in one place would be a good idea. Please comment the websites you find useful :D
List as of now:
- [CF-Predictor](https://cf-predictor-frontend.herokuapp.com/) ([Original Blog](https://codeforces.me/blog/entry/50411)): Predicts the expected rating change during and after contests.
- [Code-Drills](https://code-drills.com/) ([Original Blog](https://codeforces.me/blog/entry/48991)): Recommends problems for a list of user & gives statistics on total number of problems solved.
- [Codeforces Visualizer](https://cfviz.netlify.com/) ([Original Blog](https://codeforces.me/blog/entry/50326)): Gives various statistics for a user, and supports comparison between two users.
- [CF Statistics](https://cfstatistics.herokuapp.com/) ([Original Blog](https://codeforces.me/blog/entry/60775)): Comparison between 2 users ...
- [CF Notifications](http://cfnotifications.com/) ([Original
Blog](https://codeforces.me/blog, - [CF-Predictor](https://cf-predictor-frontend.herokuapp.com/) ([Original
Blog](https
163.
Последние нововведения на Codeforces [Январь 2021] Всем привет!
Команда Codeforces усердно трудится каждый день над улучшением платформы. Вот краткий обзор последних нововведений.
### Удаление мэшапов
До недавнего времени на Codeforces не было возможности полностью удалить созданные вами мэшапы — они оставались "висеть" в интерфейсе и могли мешать, если их много.
Теперь же вы можете удалить созданные вами мэшапы. Для этого перейдите на страницу редактирования:
В самом низу страницы будет находится кнопка для удаления:
Удаление мэшапа обратимо, то есть вы сможете восстановить его при необходимости. Просмотреть удаленные мэшапы можно нажав на ссылку ниже таблицы с мэшапами.
### Другие нововведения
- Добавлен фильтр по формату тренировки (IOI или ICPC) в списках мэшапов и тренировок.
- Обновлены версии компиляторов: Python 2 ...
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