Rubikun's blog

What were your favorite problems in 2025?
By Rubikun, 9 months ago, In English

Hello, Codeforces !!

I think that in some countries, 2026 has already begun, but I felt that I hadn’t seen this topic this year (sorry if I just missed it), so I hurriedly made this blog post.

I’m looking forward to seeing lots of interesting problems! (of course, non-Codeforces problems are also OK)

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9 months ago, hide # |
 
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these will probably seem like middling picks in retrospect, but they're the only ones on the top of my mind right now

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As I started new, these are my favourites:

Watermelon (helped me start) Way too long words (my first own solved problem with no help) bit++ (took a long time but was solved)

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Palindrome Flipping because it was the first 2000+ rated problem I solved in-contest: https://codeforces.me/problemset/problem/2158/D

The solution I found, which was to reduce both the source and target strings to canonical form, was very appealing to my mathematician senses.

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My top picks.

  1. Hacking numbers
    2. All the 3 versions of the problems are really interesting.
  2. For the Champion
    3. great idea.
  3. Segments Covering.
    4. Cool dp+probablity.
  4. Flatten or Concatenate
    5. This is from the goodbye 2025. The idea and proof is awesome.

ps: The list may feel like biased as I really like adhoc and constructive types problems.

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Cute problems

Smashable problems

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I don't know whether these problems are interesting for GMs but for me they were one of the best problems I have ever solved on codeforces:

Minimizing the difference : https://codeforces.me/contest/1244/problem/E

Big Brush : https://codeforces.me/problemset/problem/1638/D

That's what I can remember right now.

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2129C3 - Interactive RBS (Hard Version) this as well as the easier versions, Very nice to figure out, beautiful problem.

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I will shamefully advertise myself. I think https://codeforces.me/contest/2124/problem/H and https://codeforces.me/contest/2124/problem/I are the best and hardest problems I've ever authored (and probably will be the best and hardest problems I ever will).

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some of my favourite problems from this year, some taught me new ideas, some other had me thinking about them for prolonged periods of time, some have very surprising solutions, because of their simplicity or because they need some creative combination.

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2128F - Strict Triangle was cool

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Lots of great interactives this year. Here's some that haven't been mentioned that I enjoyed:

Other problems I liked a lot:

Definitely forgetting many great problems, might add more later.

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These are my favourites:

  1. Trails (Hard) (Beautiful math)
  2. Removal Game (A friend asked me for the solution, but nobody in the room understood my explanation :()
  3. Mr. Kitayuta's Technology (Love how the author tricked me through example images)

Happy coding!

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https://codeforces.me/contest/2164/problem/B the most beautiful div2B I have ever solved

https://codeforces.me/contest/2113/problem/C This problem had an amazing visualisation to get the solution

I remember just going “OHHHHHHHHH” when I got the fullsolve idea for both of these problems, and subsequently having a really good mood throughout the rest of the day

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My favourite problems:

1798E - Multitest Generator

2144E2 - Looking at Towers (difficult version)

2134E - Power Boxes

1738D - Permutation Addicts (the worst on this list because of my reading comprehension)

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I might get a bit of hate but it was from Atcoder.

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I'll exclude my own problems, because I like all of them :)

One problem from each difficulty 800-2200
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I didn't solve enough problems to have a favorite group of problems, but I really liked the problems from https://codeforces.me/blog/entry/141935 (Neowise Labs), set by SanguineChameleon, thenymphsofdelphi, and coordinated by the GOAT himself 244mhq

Thanks to all setters and organizers for a great year <3

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2128C because it was the first C i ever solved, from CSES 1637, 1692, 1651, 3304, 1145

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Maple and Tree Beauty

Even though the contest got unrated, I loved it. I remember that before solving this I was at a -120 and after this I was at -10 and I was so happy that I was literally saying I don't care about the -ve rating lol

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1453B - Suffix Operations

I was literally stuck on the solution for 4 hours straight, I'm currently rated 1000 and this is 1400 rated prob. Constructive and Implementation Tags

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I really liked the first 3 tasks in Good Bye 2025 and the first 4 in the latest Educational one. There was what I'm currently studying, dynamic programming, prefix sums, combinatorics, and these competitions also brought me into these topics.

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https://codeforces.me/contest/2172/problem/H is a pretty fun exercise in the sense that you should look at all strings being compared and not just a pair.

https://codeforces.me/contest/2155/problem/F in the sense it's just 506D in a different coating.

https://codeforces.me/gym/105928/problem/E for being one of the most painful problems to implement.

https://codeforces.me/contest/2153/problem/F for being very similar to 1771F. (Also I solved it in the contest)

https://codeforces.me/contest/1771/problem/F for being very similar to 2153F. (Also it's a pretty fun hashing problem too)

https://codeforces.me/contest/997/problem/E for being the embodiment of "if the problem isn't hard enough, add queries to it"

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A few problems I really liked doing in 2025 (and one from 2025 that I did today):

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https://codeforces.me/problemset/problem/2119/D

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https://codeforces.me/contest/2173/problem/C my first time solving a 1400 rated problem in a contest

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These problems really felt educative and i liked it : 2176D - Fibonacci Paths 2175D - Wishing Cards 2145D - Inversion Value of a Permutation 2107D - Apple Tree Traversing

Also, I would love to see these kinds of problem appear a little more this year : 2146D2 - Max Sum OR (Hard Version) 2149G - Buratsuta 3 2111E - Changing the String

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This was probably the problem I liked the most:

I spent an entire day thinking on this, and at the end I drew inspiration from tetra valency of carbon in organic chemistry to solve it.

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380C - Sereja and Brackets

1108E2 - Array and Segments (Hard version)

First time i used Segtree here

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IOI 2025 Worldmap was pretty cool!

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it was really intresting to solve this one and also took a while to solve Errich-Tac-Toe (Easy Version)

still figuring out the hard one Errich-Tac-Toe (Hard Version)

it was global round 12 but still i found this last yr only :cry:

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I liked the problem 2167A - Square? by Intellegent

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https://www.luogu.com.cn/problem/P11831

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AGC073-A Chords and Checkered

Especially the solution given by Um_nik, which is so beautiful

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https://www.luogu.com.cn/problem/P14636

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2178G - deCH OR Dations

A good XOR problem.

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