أهلاً, Codeforces!

cry, Intellegent, and I are really excited to invite you to participate in Codeforces Round 1029 (Div. 3), which will take place on Jun/08/2025 17:35 (Moscow time). You will be given $$$2$$$ hours and $$$15$$$ minutes to solve $$$8$$$ problems.

The round will be hosted by rules of educational rounds (extended ICPC). Thus, all solutions will be judged on preliminary tests during the round, and after the round, there will be a 12-hour phase of open hacks. After the open hack phase, all accepted solutions will be rejudged on successful hacks. Also, note that there is no score distribution but the usual penalty of 10 minutes for each wrong submission, following the rules of educational rounds.

Note that the penalty for each wrong submission in this round is 10 minutes. Also, note the rule restricting AI use. If you are caught using AI in an unorthodox manner, you will be sent to cry's basement. You don't want that to happen.

Remember that only the trusted participants of the third division will be included in the official standings table. As it is written by link, this is a compulsory measure for combating unsporting behavior. To qualify as a trusted participant of the third division, you must:

• take part in at least five rated rounds (and solve at least one problem in each of them)
• do not have a rating of 1900 or higher at any moment in time.

Regardless of whether you are a trusted participant of the third division or not, if your rating is less than 1600, then the round will be rated for you (unless you register unrated).

I would like to thank the following people for making this round possible:

• cry for being an amazing coordinator and being even more amazing to add a problem to the set, and Intellegent for adding a wowee problem to the set.
• Dominater069 for red testing.
• Proof_by_QED, __baozii__, AksLolCoding, amoeba4, catgirl, efishel, -firefly-, Edeeva, 18o3 for orange testing.
• reirugan, Friedrich, wuhudsm, rewhile, IceWolf898 for purple testing.
• macaquedev, beaten_by_ai, SpyrosAliv, chromate00, Non-origination, expertaq, DivinePunishment for blue testing.
• Borhom, _Rawan_, ETL for cyan testing.
• hotfog12 for grey testing.
• rlin61 for unrated testing.
• Vladosiya for statement translation and testing as well.
• MikeMirzayanov for developing Codeforces and Polygon.

Good luck, and most importantly, have fun!

Edit: Tutorial

Hello Codeforces!

The series of Educational Rounds continues thanks to the support of the Neapolis University Pafos.

On Jun/03/2025 17:35 (Moscow time) Educational Codeforces Round 179 (Rated for Div. 2) will start.

This round will be rated for the participants with rating lower than 2100. It will be held on extended ICPC rules. The penalty for each incorrect submission until the submission with a full solution is 10 minutes. After the end of the contest, you will have 12 hours to hack any solution you want. You will have access to copy any solution and test it locally.

You will be given 6 or 7 problems and 2 hours to solve them.

Almost all of the problems in the round were invented and prepared by me. I would like to thank Ivan BledDest Androsov for the problem that made the round more balanced.

And once again, I would like to express my sincere thanks to the round coordinators: Ivan BledDest Androsov and Mikhail awoo Piklyaev for improving the quality of the problems and helping with their preparation.

And of course, huge thanks to Mike MikeMirzayanov Mirzayanov for great systems Polygon and Codeforces.

Good luck to all the participants!

Our friends at Neapolis University Pafos also have a message for you:

Admission to the Computer Science and Artificial Intelligence Bachelor’s program at Neapolis University Pafos is ongoing!

The JetBrains Foundation is offering 20 fully funded scholarships for talented first-year students.

Each scholarship covers the entire duration of the program, including tuition, accommodation, medical insurance, visa fees, and €300 per month for personal expenses.

Good news! If you missed the first round, there’s still time to apply for the second round! Submit your documents, pass the entrance test and interview, and receive a full scholarship.

• Application deadline – June 11, 2025
• Entrance test – June 15, 2025 (this is the last entrance test for 2025!)

Additionally, there are 2 fully funded scholarships available for students wishing to transfer into the second year of the program (for those already studying Computer Science).

If you have any questions, feel free to ask in the Telegram chat or email us at [email protected] !

UPD: Editorial is out

Hello, Codeforces!

I'm pleased to invite you to Codeforces Round 1028 (Div. 1) and Codeforces Round 1028 (Div. 2). It starts on 31.05.2025 17:35 (Московское время). This means that Children's Day will come during this round. I'm sure everyone will be impressed with Children's Day, even if they're no longer children.

I remember when I was a kid, I always looked forward to Children's Day. On Children's Day, there were always candies and fun activities at school. But as I grew up, this festive atmosphere was diluted by the trivialities of life. But luckily, we had Codeforces. Spending the holidays with interesting problems doesn't actually have to be more boring than candies and activities ¯\_(ツ)_/¯

This will be the second round I've hosted on Codeforces. To make it better, this time I've called on my friend MagicalFlower to help me organize this round. Also, errorgorn has helped us very much, we are fully indebted to this well known 🐸 on Codeforces!

For both divisions, you will have 2 hours to solve 6 problems. I hope you will enjoy these problems.

I would like to thank:

• errorgorn for his 🥵 and 🥶 coordination.
• Alexdat2000 for Russian translation.
• MagicalFlower for providing more rejected problems than me. While you may not be able to see these problems, these efforts cannot be ignored. 👈🤣
• EvenImage for existing and VIP Testing.
• JoesSR, qiuzx and 275307894a for Legendary Grandmaster Testing.
• zhaohaikun, N_z__, HealTheWorld, _istil, grass8sheep, EasonTAO and ffao for International Grandmaster Testing.
• Error_Yuan, H_W_Y, Hanghang007 and literalchild for Grandmaster Testing.
• tarche for International Master Testing.
• ODT, Tyx2019, Hank2019, Bazoka13 and bookcat for Master Testing.
• Shoryu386 and ToxicPie9 for Candidate Master Testing.
• daitangchen2008 for Specialist Testing.
• zhufangyi, ls114514, clare_288, Awd_, wengxuanyu, likerun and JessicaZhang for Newbie Testing.

• You for participating the round.

• MikeMirzayanov and KAN for Codeforces and Polygon systems.

Especially, I'd like to thank JoesSR, zhaohaikun and ToxicPie9 for helping us prepare this round, which might not be able to run as smoothly as now without them.

Finally, I would like to give my heartfelt thanks and praise to errorgorn.

Both of the rounds I hosted would not have been possible without the support and efforts of errorgorn. Although he often rejected my problems 💀 and stood me up a lot 😡, I could sense his love for the problems and his seriousness about the competition in the time we spent together preparing for these rounds.

errorgorn is an interesting guy, you can often see him sending funny emoji like 🤯 in the discord, and he would also sometimes share me with some of the local culture of Singapore. I am so lucky to have had this fun time with him.

Some time ago, errorgorn happened to tell me that he might not have time to continue as Coordinator, which I deeply regretted. Likewise, I'm starting my college career, and this will probably be the last round I host at Codeforces as well.

Hopefully this round will be a perfect end to my experience hosting rounds on Codeforces. It's really quite an unforgettable experience, and I'm also looking forward to the next collaboration with errorgorn.

The main character of the problems will be Gellyfish🍏, and her friend Flower🌸.

Score distribution:

• Div.1: $$$500-1250-1750-2250-3000-(2500-3500)$$$
• Div.2: $$$500-750-1250-2000-2500-3000$$$

Good Luck & Have Fun! 🔥🔥🔥

UPD1: Congratulations to the winners!

UPD2: Editorial is out.

Hello, Codeforces!

I have a short update. With the help of Kleidemos, we've improved C# support. You can now use the modern dotnet-9, and the C# language version has been upgraded to 13. Additionally, the F# language (version 9) is now supported as well. Both compilers are currently in experimental mode. Once we make sure everything works smoothly, I'll remove one of the older C# versions and leave only two.

Hi Codeforces!

I’ve long believed that it's really worth revisiting the same competitive programming topics multiple times. The first time, you’re just trying to survive and maybe grasp the basics. The second time, you’ve solved some problems, you know where you got confused, and you're ready to absorb more. By the third time, you’ve seen and solved enough to start appreciating the subtleties — the tricks, advanced applications, deeper meaning. But most lectures I’ve seen are designed as a one-size-fits-all — trying to cover everything from scratch and dive into the advanced stuff in one go. That’s rarely optimal.

Take binary search. I once talked about it for six hours straight. A beginner at that lecture would’ve probably quit CP immediately. And that’s ok. Beginners don’t need all that. But the truth is, there is that much depth to explore even in seemingly simple topics like binary search — once you're ready. Yet many people only see the beginner version once and move on. Their understanding stays stuck at that level forever.

FFT is another great example. Almost every FFT lecture I’ve seen tries to do everything in one sitting: start from zero and end with "you now know everything there is to know about FFT and all its applications". But FFT is big, it’s hard. It makes more sense to have:

• A basic lecture: just enough to multiply polynomials with minimal pain.

• An intermediate one: assume you know the basics, and dive deeper.

• And maybe an advanced one: weird tricks, optimizations, cool use cases.

There are many more topics like that: segment trees, square root decomposition, MSTs, etc.

So I’m starting an irregular stream series called "Algorithms in Depth". Each topic will be split by level:

• Beginner: minimum viable understanding to start solving problems.

• Intermediate: expanding the basic understanding to solve harder problems.

• Advanced: deep understanding, fun tricks, and higher-level abstractions.

Most lectures try to combine the beginner and intermediate levels and skip the advanced part completely. I want to keep these levels separate — to not overwhelm beginners, bore intermediates, or neglect the advanced participants. I'm starting with Fast Fourier Transform (FFT). The first stream will cover only the basics. No fancy math. Just the simplest path to fast polynomial multiplication. As much motivation and intuition as possible. If you know what a polynomial is and how to multiply them by hand, you're good to go.

The stream will happen on my youtube channel on Thursday, May 29, 2025 at 14:00 UTC. Everyone’s welcome — ask questions in chat, and the recording will be available afterward. Hope to see you there!

Hello! Codeforces Round 1027 (Div. 3) will start at May/26/2025 17:35 (Moscow time). You will be offered 7 problems with expected difficulties to compose an interesting competition for participants with ratings up to 1600. However, all of you who wish to take part and have a rating of 1600 or higher, can register for the round unofficially.

The round will be hosted by rules of educational rounds (extended ICPC). Thus, solutions will be judged on preliminary tests during the round, and after the round, it will be a 12-hour phase of open hacks. After open hacks all accepted solutions will be rejudged on successful hacks.

You will be given 7 problems and 2 hours and 15 minutes to solve them.

Note that the penalty for the wrong submission in this round is 10 minutes.

Remember that only the trusted participants of the third division will be included in the official standings table. As it is written by link, this is a compulsory measure for combating unsporting behavior. To qualify as a trusted participant of the third division, you must:

• take part in at least five rated rounds (and solve at least one problem in each of them)
• do not have a point of 1900 or higher in the rating.

Regardless of whether you are a trusted participant of the third division or not, if your rating is less than 1600, then the round will be rated for you.

Problems have been created and written by our team: myav, Gornak40, ibraevdmitriy and Vladosiya.

We would like to thank:

1. MikeMirzayanov for Polygon and Codeforces platforms.

2. imirdy for red testing.

3. Be_dos, KseniaShk, pengin_2000, Eugene312 for yellow testing.

4. Kmes for purple testing.

5. RobinFromTheHood, AlphaMale06, Kosya, itz_pabloo, umezo, donovilia2007, l-_-l for blue testing.

6. rcht, zoro2000, ne_justlm, IceHydra for cyan testing.

7. jbrenorv for green testing.

Good luck!

UPD: Editorial is out!

Hello, codeforces!

We are glad to invite you to Codeforces Round 1026 (Div. 2), which will start at May/24/2025 17:35 (Moscow time). This round will be rated for all participants with a rating below 2100. You will have 2 hours to solve 6 problems. The problems were prepared by XaRDKoDblCH and PvPro.

We would like to thank everyone who made this round possible:

• FairyWinx, for coordinating our round and preparing the problems.

• MikeMirzayanov, KAN for the Codeforces and Polygon systems.

• Our army of testers: N_z__, BurnedChicken, triple__a, makrav, Noobish_Monk, mainyutin, k1r1t0, Nil2007, zhiganov_v, sputn1k, lewc, larionov_kirill, Kmes, Dimozavr, potalov.m, Mega_Plus, larush, mz1.

• All the participants.

Score distribution: 500 — 750 — 1500 — 2000 — 2250 — 3000

Our round will be dedicated to a cyberpunk theme, so get ready to save the world from robots! ;)

Good luck!

UPD: The contest is over! Congrats the winners:

via all participants:

1. maspy

2. Geothermal

3. 9ovem

4. peti1234

5. turmax

via div.2 participants:

1. 9ovem

2. still_still_stellar

3. Hellia

4. Badint

5. cuongaaaa

UPD: Editorial

Hello everyone!

Since recently I have rejoined Codeforces dev team. I worked on it before but had a break for some years.

There is always a lot of work happening behind the scenes, invisible to the users, but sometimes it is nice to have visible updates on the website.

Today, we have a couple improvements for the comment section.

Changing/cancelling votes

It happens sometimes that you click a wrong button, which can be annoying when voting for comments. Now it is possible to change your vote, or cancel it (but only during the first minute).

Hiding comments

When reading comments it is sometimes useful to hide the comments. Maybe you have already read them, or you don't care about replies to a particular comment. So now you can toggle comment tree visibility with a single click.

If you have an idea of a small and simple improvement for Codeforces, let us know in the comments!