Tanzim_bn's blog

Problem of the Year 2024
By Tanzim_bn, history, 7 weeks ago, In English

As 2024 nears its end, let’s share the problems we enjoyed the most this year so far! Drop the problem name, link and what made it special (optional).

  • Vote: I like it
  • +82
  • Vote: I do not like it

»
7 weeks ago, # |
  Vote: I like it +29 Vote: I do not like it

2044F - Easy Demon Problem Quite an interesting problem, looked really hard but had an easy mathematical solution (Do give it a try if you haven't).

→ Reply
»
7 weeks ago, # |
  Vote: I like it +30 Vote: I do not like it

1933E. Xor-Grid Problem It demonstrates the magical nature of XOR operations, completing the missing rows and columns in an array.

→ Reply
»
7 weeks ago, # |
  Vote: I like it +20 Vote: I do not like it

1919D — 01 Tree Everything fits so nicely together!

→ Reply
»
7 weeks ago, # |
  Vote: I like it +9 Vote: I do not like it

1878F - Vasilije Loves Number Theory

→ Reply
»
6 weeks ago, # |
  Vote: I like it +7 Vote: I do not like it

2029C - New Rating Although dp is easier to think here, the bin search solution is really satisfying.

→ Reply
»
6 weeks ago, # |
  Vote: I like it +4 Vote: I do not like it

1843E — Tracking Segments binary search on the prefix sum!

→ Reply
»
6 weeks ago, # |
  Vote: I like it +3 Vote: I do not like it

https://codeforces.net/contest/1931/problem/F It was such a nice thing to use cycle detection in a statement like this.

→ Reply
»
6 weeks ago, # |
  Vote: I like it +52 Vote: I do not like it

Universal Cup 3.11 O - New School Term

I enjoyed being trolled by this problem. The final observation is so natural, yet so invisible. As far as I know, some LGMs were also trolled.

→ Reply
  • »
    »
    6 weeks ago, # ^ |
      Vote: I like it +16 Vote: I do not like it

    Can you share the observation? My idea is a bit ugly.

    Spoiler
    → Reply
    • »
      »
      »
      6 weeks ago, # ^ |
        Vote: I like it +16 Vote: I do not like it
      Spoiler
      → Reply
      • »
        »
        »
        »
        6 weeks ago, # ^ |
          Vote: I like it +16 Vote: I do not like it

        I feel so stupid now ;-; Thanks!

        → Reply
      • »
        »
        »
        »
        6 weeks ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        do you know how can i participate in this cup ? and if iam not a high rated one can i ?

        → Reply
»
6 weeks ago, # |
← Rev. 2   Vote: I like it +23 Vote: I do not like it

1977D - XORificator never seen this trick before and thought it was unsolvable before seeing the tutorial

→ Reply
»
6 weeks ago, # |
  Vote: I like it +3 Vote: I do not like it

https://codeforces.net/problemset/problem/2042/E

→ Reply
»
6 weeks ago, # |
← Rev. 2   Vote: I like it +9 Vote: I do not like it

ARC171E — Rookhopper's Tour

The setting feels very chaotic to me initially, but after thinking it though, it turns out there exist an elegant way to analysis the problem model very simply to make it solvable. It's just... astonishing!

→ Reply
»
6 weeks ago, # |
  Vote: I like it +12 Vote: I do not like it

https://codeforces.net/contest/1991/problem/H

→ Reply
»
6 weeks ago, # |
  Vote: I like it -24 Vote: I do not like it

D. YEET!

Not because I'm the author , but because the non-randomized solution is really satisfying and cool !

For Problems I solved , hmm

B. Shohag Loves Strings is cool.

D. Harder Problem is indeed a nice "harder" trolling problem .

→ Reply
»
6 weeks ago, # |
  Vote: I like it -62 Vote: I do not like it

https://codeforces.net/contest/321/problem/C

My 1st and only centroid decomposition AC

→ Reply
»
6 weeks ago, # |
  Vote: I like it +3 Vote: I do not like it

I don't know about best problem, but worst problem of the year is definitely problem C in APIO, the problem sucks A LOT

→ Reply
»
6 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

All of the median related problems which appeared in late 2024

→ Reply
»
6 weeks ago, # |
  Vote: I like it +1 Vote: I do not like it

IOI 2024 Tree was nice.

→ Reply
»
6 weeks ago, # |
  Vote: I like it +18 Vote: I do not like it

2034F2 - Khayyam's Royal Decree (Hard Version) deepened my understanding in combinatorics, I found the connection to Principle of Inclusion-Exclusion especially nice

→ Reply
»
6 weeks ago, # |
  Vote: I like it +36 Vote: I do not like it

1930G - Prefix Max Set Counting. After you realize that there's a natural ordering of the subtrees based on their max element, the way the solution comes together is incredibly satisfying--solving the problem from there felt like entering a flow state. Thanks to satyam343 for the great problem!

I think I forgot to post problems for the last few years, so I'll share belated choices for 2023 and 2022. My favorite problem of 2023 by far was 1889D - Game of Stacks. The cycle observation was incredible--as I wrote after the contest, "I spent several minutes thinking to myself about how cool that observation is before going to implement." Stylistically, I also enjoyed that the problem really was just about that one observation--there was one idea, that idea turned out to be really cool, and then it wasn't bogged down in unnecessary details or implementation. Thanks, waaitg!

I don't have as clear of a favorite for 2022, but I'm pretty partial to USACO Hoof and Brain. It was a very tricky problem solved by relatively few students in a five-hour contest, but the solution is simple enough that I think it could be explained to someone with essentially no competitive programming experience. It's also a nice application of the strategy of identifying trivial cases and removing them in order to impose new constraints on the input that turn out to make the problem easier. Thanks, Tlatoani!

→ Reply
  • »
    »
    6 weeks ago, # ^ |
      Vote: I like it +10 Vote: I do not like it
    I don't have as clear of a favorite for 2022, but I'm pretty partial to USACO Hoof and Brain. It was a very tricky problem solved by relatively few students in a five-hour contest, but the solution is simple enough that I think it could be explained to someone with essentially no competitive programming experience.

    Funny thing actually. I distinctly recall discussing this problem with nickbelov when he was only 2 months into competitive programming / contest math stuff in general. He understood basic game theory and derived the high level solution by talking about "merging clouds" of states which is precisely what the editorial solution is. The only issue was implementation details, since I don't think he's seen that sort of DSU direct graph-contraction trick before, and was stuck on using BFS to implement his idea lol.

    So he is living proof that it can be explained to someone (and even solved) by someone with essentially no competitive programming experience :)

    → Reply
»
6 weeks ago, # |
  Vote: I like it +7 Vote: I do not like it

2029E - Common Generator Interested me so much that I spent my entire day just before my end semester exams solving it (only 2100 I have ever solved on my own till now)

→ Reply
»
6 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

I personally like solving constructive algo problems. Not sure about which question I enjoyed the most in the year but recently I enjoyed solving https://codeforces.net/contest/2040/problem/C and https://codeforces.net/contest/1375/problem/D

→ Reply
»
6 weeks ago, # |
  Vote: I like it +4 Vote: I do not like it

Swap columns and find a path

This problem shows how greedy approaches can be wrong.

Also, try to solve it in O(N) complexity.

→ Reply
»
6 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

966E - May Holidays The problem that combines my two favourite techniques (HLD and sqrt decomp). I had great fun solving it.

→ Reply
»
6 weeks ago, # |
  Vote: I like it +10 Vote: I do not like it

Some of my favorites include

1935E - Distance Learning Courses in MAC

1965E - Connected Cubes

1977E - Tensor

1991G - Grid Reset

Of course, I'm a little biased but I also really like 2035D - Yet Another Real Number Problem, 2035F - Tree Operations, and especially 2035H - Peak Productivity Forces from global round 27. :)

→ Reply
»
6 weeks ago, # |
  Vote: I like it +3 Vote: I do not like it

2042E - Vertex Pairs

→ Reply
»
6 weeks ago, # |
  Vote: I like it +3 Vote: I do not like it

ARC173D

This one was very cute.

AGC069B

My thoughts

Stone Catch Game

This one isn't from 2024, but this is just a really neat problem. The solution "makes so much sense" but like, as for how you come up with it, is kind of insane. I went down so many seemingly correct rabbitholes in contest for this.

→ Reply
»
6 weeks ago, # |
  Vote: I like it +11 Vote: I do not like it

2029I - Variance Challenge — I think this is the ever best problem of mine!

→ Reply
»
6 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

https://codeforces.net/contest/2030/problem/D Really enjoyed solving this during the contest!

→ Reply
»
6 weeks ago, # |
  Vote: I like it +14 Vote: I do not like it

AtcoderWTF2024_B

→ Reply
»
6 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

https://codeforces.net/contest/1624/problem/B

→ Reply
»
6 weeks ago, # |
  Vote: I like it +3 Vote: I do not like it

AddMulExp from WWPIT, set by CutSandstone: $$$f(x, n) = 1x^1+2x^2+3x^3+...+nx^n$$$, given $$$x$$$, $$$n$$$, and $$$m$$$, find $$$f(x, n)\bmod m$$$. The solution is very slick!

→ Reply
»
6 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

personal problem list:

Tree chain decomposition： https://codeforces.net/contest/2028/problem/E

dp questions： https://codeforces.net/contest/2028/problem/D , https://codeforces.net/problemset/problem/2025/D , https://codeforces.net/problemset/problem/2041/C

High-Precision： https://codeforces.net/gym/105540/problem/E

graph： https://codeforces.net/contest/2014/problem/E

learning data structures(G1 to G3)： https://codeforces.net/contest/2009/problem/G1

magic usage of multiset： https://codeforces.net/contest/2008/problem/E

→ Reply
»
6 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

1994D - Funny Game

2022D2 - Asesino (Hard Version)

1919F2 - Wine Factory (Hard Version)

1975E - Chain Queries

→ Reply
  • »
    »
    6 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    oh, also reason

    1994D the way to construct using pigeonhole principle is really fun.

    2022D2 I got trolled in optimizing the number of queries when $$$n=3,5,7$$$

    1919F2 using segment tree to calculate maximum flow is amazing

    1975E I learned the trick to check a chain

    → Reply
»
6 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

https://codeforces.net/contest/2009/problem/F Really loved formulating the answer here. When the solution went from "Running test" to "Accepted", it was definitely one of my happiest moments of Codeforces this years. I don't know why but I just felt proud of solving this question. Here is my solution https://codeforces.net/contest/2009/submission/285274394

→ Reply
»
6 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

1993E - Xor-Grid Problem Great task.

→ Reply
»
6 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

2035F - Tree Operations

2040E - Control of Randomness (have a 2d dp solution to precompute all queries but, it also have a super easy O(nq) solution)

2033G - Sakurako and Chefir

2028E - Alice's Adventures in the Rabbit Hole

1714F - Build a Tree and That Is It

→ Reply
»
6 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Evacuation plan (IZhO18_plan) It uses very interesting tactics, that I've never seen before, it opened me to the advanced part of cp.

→ Reply
»
6 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

2048D — Kevin and Competition Memories It was an interesting problem with extensive implementation, but incredibly satisfying when it got accepted.

→ Reply
»
6 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

ARC186A An amazing constructive problem.

→ Reply
»
6 weeks ago, # |
  Vote: I like it +10 Vote: I do not like it

2029E for sure.

Solving this problem step by step gives me more satisfaction than I've ever had before.

→ Reply
»
6 weeks ago, # |
  Vote: I like it +10 Vote: I do not like it

https://dmoj.ca/problem/ioi24p2

great problem. final observations are really nice.

→ Reply
»
5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

I might be biased cuz I just saw it yesterday, but I enjoyed Eliminating Balls with Merging: https://codeforces.net/problemset/problem/1998/E1

→ Reply
»
5 weeks ago, # |
  Vote: I like it +10 Vote: I do not like it

UCS

→ Reply
»
5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

https://codeforces.net/contest/2053/problem/E This was the problem in yesterday's contest. I am usually afraid of solving problem >=D (D,E..). However today I was adamant that I will sit at this problem no matter how long it takes. And boom... at first glance what seemed so difficult was doable in about 2 hours.. Loved it. Wish to solve tougher problems in 2025.

→ Reply
»
5 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

https://codeforces.net/problemset/problem/2033/G

→ Reply
»
5 weeks ago, # |
  Vote: I like it +18 Vote: I do not like it

I quite liked 1973D - Cat, Fox and Maximum Array Split.

Another one would probably be problem C from Serbian OI 2024 Day 1.

→ Reply
»
5 weeks ago, # |
  Vote: I like it +39 Vote: I do not like it

I nominate 2053I2 - Affectionate Arrays (Hard Version) — just look at how much discussion it generates, must be an interesting problem :P

→ Reply