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 Ari

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 244mhq

1586F - Defender of Childhood Dreams by gotexans

1610H - Squid Game by Tet and AliShahali1382

• AGC056 — C, 01 Balanced by maroonrk
• COCI 2021/22, Round #2, Hiperkocka by keko37, Bartol, paula, pavkal5, ominikfistric, Shtef (who?)
• GR15 — G, A Serious Referee by cip999
• 2020/21 ICPC Latin American Regional — G, Game of Slots by arthur.nascimento

1586I - Омкар и мозаика by rabaiBomkarBittalBang

1537F - Figure Fixing

1589D - Guess the Permutation

Here is my favourite problems (July ~ Dec):

interesting problems

• CF1622F Quadratic Set by Neon
• CF1610F Mashtali: a Space Oddysey by AliShahali1382
• CF1612F Armor and Weapons by BledDest
• CF1605F PalindORme by ExplodingFreeze and antontrygubO_o
• CF1586I Omkar and Mosaic by rabaiBomkarBittalBang
• CF1592F2 Alice and Recoloring 2 by Bakry, Hemose and KostasKostil
• CF1567F One-Four Overload by flamestorm
• CF1562F Tubular Bells by Wind_Eagle
• CF1548E Gregor and the Two Painters by Benq
• CF1552H Guess the Perimeter by cip999 and dario2994
• CF1553H XOR and distance by 244mhq
• CF1545C AquaMoon and Permutations by AquaMoon
• CF1545D AquaMoon and Wrong Coordinate by AquaMoon
• CF1543E The Final Pursuit by JaySharma1048576

"trick" problems

• CF1606F Tree Queries by BledDest
• CF1578G Game of Chance by ?
• CF1580F Problems for Codeforces by CQXYM
• CF1553I Stairs by bthero, 244mhq, BledDest

educational problems:

• CF1558F Strange Sort by tourist
• CF1548D2 Gregor and the Odd Cows (Hard) by Agnimandur
• CF1530H Turing's Award by tourist
• CF1545F AquaMoon and Potatoes by ODT and box
• CF1542E2 Abnormal Permutation Pairs (hard version) by gyh20 and feecIe6418

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

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!).

1583F - Defender of Childhood Dreams Like: Dislike:

Happy holidays Anton!

My favorite problems that you authored this year are:

• AGC055E (Set Merging)
• G̶P̶ ̶o̶f̶ ̶I̶M̶O̶ ̶C̶ ̶(̶C̶r̶a̶b̶'̶s̶ ̶C̶a̶n̶n̶o̶n̶)̶
• Oct Cookoff PERMUTINVS

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

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

• 1617D2 - Too Many Impostors (hard version)
• 103148C - Twin Cookies
• 1516D - Cut
• 1513C - Add One
• 1505E - Cakewalk (from April Fools Day Contest 2021)
• 1491F - Magnets (my favourite)
• 1451D - Circle Game (edit: I just realised that this problem is from late 2020. Anyway, still a very good problem)

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

• GP of Nizhny Novgorod H
• IOI 2021 Keys

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:

• https://codeforces.me/contest/1610/problem/D

2100:

• https://codeforces.me/contest/1606/problem/E

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:

• Final Exam, from Petrozavodsk Winter 2021 PKU Contest
• Rectangle Painting, from Petrozavodsk Winter 2021 NNSU Contest (XXI Open Cup GP of Nizhny Novogorod)
• Build The String, from XXI Open Cup GP of Urals
• Slide Circuits from Google Code Jam 2021
• Joy with Permutations, from Petrozavodsk Summer 2021 Kyoto U Contest (KCPC 2020)
• K-onstruction, from Petrozavodsk Summer 2021(XXII Open Cup GP of IMO)
• Pac-Man Speedrun from RuCode 4.0 Championship
• Interesting Numbers from AMPPZ 2021 (MW Day 1, XXII Open Cup GP of Poland)
• Short Coding from ICPC Yokohama Regional 2020 (in 2021)
• Squid Game from ICPC Seoul Regional 2021
• 1%-Euclidean from ICPC Moscow Regional 2021
• Areny from Potyczki Algorytmiczne 2021, Runda 4
• Drzewo czerwono-czarne from Potyczki Algorytmiczne 2021, Runda 5

Paired Up by Benq

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

• AGC52 B and C by antontrygubO_o
• JOISC21-Ancient Machine
• ARC119-D

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:

• 1491G - Switch and Flip, an interesting constructive problem requiring thinking.
• 1491H - Yuezheng Ling and Dynamic Tree, a classical model but also new in this problem.
• 1500C - Matrix Sorting, the transformation to graph theory is amazing.
• 1499G - Graph Coloring, a good education problem on euler's circuit.
• 1503D - Flip the Cards, a perfect problem requires observation.
• 1508D - Swap Pass, interesting combination of geometry and graph theory.
• 1517F - Reunion, a brand new problem requires thinking, although its tricks in tree DP and combinatorics are common.
• 1534F2 - Falling Sand (Hard Version), good problem on graph theory and conclusion-observation.
• 1540E - Tasty Dishes, really amazing linear algebra problem!
• 1530H - Turing's Award, a brilliant problem bringing me new thoughts on LIS.
• 1552G - A Serious Referee, interesting conclusion and unexpected time-complexity.
• 1558F - Strange Sort, a problem about sorting which seems common but actually has plenty to observe and.
• 1603D - Artistic Partition, combines number theory and common DP optimizing skills.

From AGC :

• AGC052C — Nondivisible Prefix Sums
• AGC055C — Weird LIS
• AGC055E — Set Merging
• AGC055F — Creative Splitting

(all from antontrygubO_o)

From Codeforces:

• 1500C — Matrix Sorting
• 1545C — AquaMoon and Permutations
• 1508F — Optimal Encoding
• 1580F — Problems for Codeforces
• 1601F — Two Sorts
• 1610G — AmShZ Wins a Bet
• 1610H — Squid Game

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

• ARC121E

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