As the last big contest of 2021 is over, I think it's time.

It seems that there were just more and more ad-hocs/"deeply analyze some complicated process" everywhere... But maybe you still liked something.

So, what were your favorite problems in 2021?

1583F - Defender of Childhood Dreams

hmm, 2021 was full of interesting tasks, so I even can't enumerate them, but these problems I remembered the most:

914G - Sum the Fibonacci

Weirdtree, RMI day 2

Чапаев на дереве

Grid and Tokens

I hope somebody will enjoy them as well as I did.

Last round's problem B :)

Codechef: Max-Min Rotations from November Lunchtime https://www.codechef.com/LTIME102A/problems/MNMXROT

Atcoder: Set merging from AGC055 https://atcoder.jp/contests/agc055/tasks/agc055_e

Codeforces: I loved Global Round 15, especially F and G were really enjoyable.

Honorable mention from other competitions: ICPC WF problem about snowblowers. Most people will think it's garbage, but I've actually enjoyed it a lot. There are at least two different clever ways to do it that are not painful.

https://open.kattis.com/problems/snoproblem

My favorite problems from Codeforces this year:

1491I - Ruler Of The Zoo by oolimry

1503F - Balance the Cards by Monogon

1508D - Swap Pass by Sexpert

1548E - Gregor and the Two Painters by Benq

1552G - A Serious Referee by cip999 and dario2994

1552H - Guess the Perimeter by cip999 and dario2994

1553H - XOR and Distance by mhq

1586F - Defender of Childhood Dreams by MagentaCobra

1610H - Squid Game by Tet and AliShahali1382

none of them are coordinated by you.

Spoilerjust kidding

1586I - Omkar and Mosaic by rabaiBomkarBittalBang

1537F - Figure Fixing

1589D - Guess the Permutation

Here is my favourite problems (July ~ Dec):

## interesting problems

## "trick" problems

## educational problems:

https://oj.uz/problem/view/APIO21_jumps

any problem from AtCoder Regular Round

All the problems become my favorites which I solved in contests in one go.

The randomized solution of 1514D

B. TMT Document by Sexpert

B. Divine Array by Anti-Light

B. Scenes From a Memory by Wind_Eagle

D. Diane by YouKn0wWho

D2. Too Many Impostors (hard version) by dbsic211

C. Baby Ehab Partitions Again by mohammedehab2002

Thank you! But did you even solve or look at the problems?

Yes, But this is my alt account, Because of some reasons I commented with my alt .. :D

Problem E from Bytedance-Moscow Workshops camp training contest 2. You had to write a randomised implementation of binary search, such that for each $$$i$$$, you need in expectation at most $$$log_2(n + 1) + 0.1$$$ queries to find $$$i$$$.

The problem is natural and interesting, and the intended solution is clean and beautiful. The problem might seem easy, but is in fact very hard (only 2 teams and none of the top 10 of the contest solved it!).

That's a 2016 problem xD

1583F - Defender of Childhood Dreams Like:Dislike:

Happy holidays Anton!

My favorite problems that you authored this year are:

Unfortunately, I don't fully understand the solutions to any of them (yet).

Thanks, but Crab's Cannon is written by gepardo :)

I solved Crab's Cannon yesterday and I also don't have an idea why!

AGC055E Set Merging by antontrygubO_o.

I am curious how can a human brain come up with such solutions ?

My favorite problem from cf : https://oj.uz/problem/view/APIO21_roads

My favorite problems are 1549D, 1602D and YATP

Mine is 1593G — Changing Brackets. At first I thought it was some kinda data structure (like segment tree). But, after reading the editorial, I didn't expect that the problem just became too mind-blowing for me because of its incredible constructive solution. How interesting!

1503C - Travelling Salesman Problem

I didn't checked out a lot of problems this year, but I really liked these two problems:

I also really liked Sorter Prodigy, but it turns out to be the best problem of 2018.

"Хорошие раскраски 2", if you know what I am talking about

1900:

https://codeforces.com/contest/1601/problem/B

https://codeforces.com/problemset/problem/1558/B

https://codeforces.com/contest/1546/problem/D

2000:

2100:

2200:

https://codeforces.com/problemset/problem/1593/G

https://codeforces.com/contest/1579/problem/G

https://codeforces.com/problemset/problem/1580/C

My favorite problems in 2021:

I'm glad to hear that, but remember that the problems from Potyczki Algorytmiczne are usually used to create Open Cup rounds, so if anybody is curious about the mentioned problems -- it's best to wait.

Paired Up by Benq

https://codeforces.com/contest/1551/problem/E by MikeMirzayanov. I really liked this problem, learnt something new

16001609D - Social Network

17001578H - Higher Order Functions

18001582F1 - Korney Korneevich and XOR (easy version)

1552D - Array Differentiation Beautiful solution

1548B - Integers Have Friends

19001482D - Playlist

20001513D - GCD and MST

21001614D1 - Divan and Kostomuksha (easy version)

1499D - The Number of Pairs

22001520F2 - Guess the K-th Zero (Hard version)

1515E - Phoenix and Computers

1485D - Multiples and Power Differences

1583F - Defender of Childhood Dreams

1542B - Plus and Multiply

1534F2 - Falling Sand (Hard Version)

ARC121D — 1 or 2

AGC056C — 01 Balanced

Here are some of my favorite problems in chronological order of their corresponding contests and the reasons I like them:

1558F — Strange Sort

1617E — Christmas Chocolates

APIO 2021 D1T1 — Hexagonal Territory

CEOI 2021 D1T2 — L-Triominoes

CNOI 2021 D2T3 — Robot

From AGC :

(all from antontrygubO_o)

From Codeforces:

i find this problem extremely beautiful , i remember sweet struggling for few days on it .

I am in process of solving New Year problems and if I have to nominate one problem from these it's definitely 41. Kingdoms and Quarantine (originally from some Summer Petrozavodsk contest). It's really great