日本語記事(For Japanese Speakers)

Previous post: https://codeforces.me/blog/entry/150057

Submission: 384239819

2026/07/26 03:13:42 JST (2026/07/25 18:13:42 UTC)

I have solved every problem in the Codeforces PROBLEMSET except for those for which I determined that obtaining an accepted verdict is impossible.

Scope

I considered the problems listed in the PROBLEMSET.

I have solved very few problems from the GYM. The only collection I worked through systematically was the PROBLEMSET.

I determined that it is theoretically impossible to obtain an accepted verdict for the following 31 problems:

775A - University Schedule, (92101 ... 92114), 927A - BuberPool Taxi Optimization, 1160A2 - Collaboration, 1576A - Communication Routing Challenge, (1639A - Treasure Hunt .... 1639K - Treasure Hunt), 1885A - Deterministic Scheduling for Extended Reality over 5G and Beyond, 1953A - Accuracy-Preserving Summation Algorithm

These problems continue to occupy the list of unsolved problems in the PROBLEMSET, and there is no way to remove them from “Last Unsolved.” Should this not be considered a bug?

Motivation

I am often asked about my motivation, but is this really such an unusual challenge that it requires a special explanation?

Personally, I think it is a very simple and universally understandable goal, comparable to the satisfaction of placing highly in a contest or watching one’s rating increase.

I believe that most users who have continued competitive programming understand the enjoyment of solving problems and learning new ideas. As a natural extension of that, I kept trying to understand and get accepted on as many problems as possible, and eventually completing the entire PROBLEMSET came within reach.

Looking back, I think I started by trying to complete all Div. 3 problems.

However, the journey was difficult. There were many problems on which I made no progress for days, and at times I doubted whether I would ever be able to finish.

Did I solve every problem by myself?

No.

I think I solved roughly 300 to 500 of the problems after reading their editorials. I have never counted them accurately, so this is only a very rough estimate.

Even when I did not read an editorial directly, there were also problems whose solutions I learned indirectly through explanations of particular techniques or discussions on social media after contests.

However, even when I read an editorial, I generally tried to understand most of the solution and its proof before implementing it myself. I did not simply copy solution code.

On the other hand, if asked whether I fully worked out a rigorous proof for every problem, the answer would not always be yes, regardless of whether I solved it independently or after reading an editorial.

For many problems, once I understood the general structure and why the solution should be correct, I considered it sufficient if it also passed randomized testing.

In particular, for some problems whose optimal structure was justified by an extremely complicated exchange argument, it was difficult to organize every detail rigorously, and I gave up on doing so completely.

Exceptional problems

There were also cases where I used methods that would not normally be allowed during a contest, including copying solution code.

• Problems for which the statement, judge, required file download links, or other necessary components were broken
• Special contests such as April Fools’ Contests
  • For many such problems, I gave up not only on solving them independently, but even on making a serious effort to understand the editorial.
• Language-restricted contests such as Kotlin Heroes
  • I also used AI to translate implementations written in C++ or another language into the required language. Doing this during a contest would clearly be cheating.
  • As far as I could determine, I found no rule prohibiting this outside a contest. However, I understand that it may not be well received by contest organizers or the competitive programming community.

For these reasons, there are aspects of this achievement that may make it inappropriate to describe it simply as "solving every Codeforces problem."

Thoughts

I believe this achievement is unprecedented, but it does not prove that I have the highest level of competitive programming skill. If a stronger user attempted the same challenge, they might well complete it in less time than I did.

This project also included many activities that do not directly translate into better contest performance. Studying obscure research-paper problems whose techniques will probably never appear again, appreciating problems with zero accepted submissions during the original contest, or becoming accustomed to spending several days debugging a single solution is unlikely to help very much in an actual contest.

Even so, I am extremely satisfied with this result.

I believe I have enjoyed the Codeforces PROBLEMSET more than anyone else.

Thank you.

A small announcement

Recently, I have started organizing solution notes, implementations, my own difficulty ratings, tags, and other information for problems I have solved, including problems from the past. This collection is not limited to the Codeforces PROBLEMSET.

I make this material available as a benefit of paid GitHub Sponsors tiers. Please check the public sample repository to see what it contains.

• Public sample
• GitHub Sponsors