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 - Правитель зоопарка by oolimry
1503F - Сбалансируйте карты by Monogon
1508D - Обмены на ребрах by Ari
1548E - Грегор и два маляра by Benq
1552G - Серьезный судья by cip999 and dario2994
1552H - Угадайте периметр by cip999 and dario2994
1553H - XOR и расстояния by 244mhq
1586F - Защитник детских мечт by MagentaCobra
1610H - Игра в кальмара by Tet and AliShahali1382
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
The randomized solution of 1514D
B. TMT Document by Ari
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 snowysecret
C. Baby Ehab Partitions Again by mohammedehab2002
Thank you! But did you even solve or look at the problems?
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: 1593G — Changing Brackets.
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.
1900:
https://codeforces.me/contest/1601/problem/B
https://codeforces.me/problemset/problem/1558/B
https://codeforces.me/contest/1546/problem/D
2000:
2100:
2200:
https://codeforces.me/problemset/problem/1593/G
https://codeforces.me/contest/1579/problem/G
https://codeforces.me/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.me/contest/1551/problem/E by MikeMirzayanov. I really liked this problem, learnt something new
1609D - Social Network
1578H - Higher Order Functions
1582F1 - Korney Korneevich and XOR (easy version)
1552D - Array Differentiation Beautiful solution
1548B - Integers Have Friends
1482D - Playlist
1513D - GCD and MST
1614D1 - Divan and Kostomuksha (easy version)
1499D - The Number of Pairs
1520F2 - 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:
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