Start with the algebraic side
The first batch opens with a linear-basis problem where the main challenge is deriving the correct counting model, not just writing the code.
3500 xor basislinear algebraweight enumeratormacwilliams identity
Codeforces 3500+ Problems
A deliberately small archive of very hard Codeforces problems, each published only after the reasoning and implementation are strong enough to stand as portfolio work. Each page keeps an original summary and links to the official Codeforces statement instead of mirroring it.
2 published deep dives
8 technique tags in use
3500+ rating floor
May 21, 2026 most recently updated page
Start here
Begin with the first verified batch
This branch stays intentionally small. Problems only appear once the derivation, correctness story, and implementation are all strong enough to publish.
Published problems
Batch 1
Every page here keeps a short original summary, a long-form derivation, pseudocode when it adds clarity, complexity analysis, and the exact C++ source file used by the page.
1336E2 3500 rating
01 Chiori and Doll Picking (hard version)
For every k from 0 to m, count how many subsets have xor with exactly k set bits.
xor basislinear algebraweight enumeratormacwilliams identity
1707E 3500 rating
02 Replace
Repeatedly replace an interval by the minimum and maximum values inside it, and find the first time it becomes [1, n].
interval dynamicsbinary liftingrange queriessparse table
Archive policy
Why this branch is intentionally small
The point of this archive is not volume. A single hard problem only ships when the explanation can show the failed naive directions, the actual turning point, and an implementation that matches the argument.
01
Statement policy
02 Original summaries only
Official statements are linked in the header of each page instead of being mirrored here.
Batch size
03 Only 2 to 5 at a time
Each batch stays small enough that the reasoning and code can be checked properly before release.
Official source
Use Codeforces for statements
The site pages are solution archives. The full problem statements remain on Codeforces.