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As Morphy hasn't posted this yet, and as the last big contest of 2020 is over, I think it's time.
2020 was a hard year for all of us, but it still gave us a lot of great contests and, luckily, no notorious coincidences. So, what were your favorite problems in 2020?
I found this problem to be cool: 1463E - Plan of Lectures. It took me around 2 hours to understand the solution and another 4 to implement, but totally worth my time!
1338E - JYPnation
E. Ski Accidents (Codeforces Global Round 8)
This problem .. I was so proud solving it during contest
1331E - Jordan Smiley
xD
1458D - Flip and Reverse
I liked Monogon's problems.
Edu 99 E. Four Points , loved those 13 WA during contest.
Also , C. Linova and Kingdom , after 11 WA during contest I reached that minimum 450 points score for the problem.
I always forget what problems I solved, so here are some good ones from my recent solves:
https://mirror.codeforces.com/contest/1406/problem/D
https://mirror.codeforces.com/contest/1451/problem/E2
This
This
Also this. Couldn't solve it during the contest (3300 is too hard for me) but I love the idea and the solution of this task.
4A - Watermelon :D
Come on, it's a joke!!!
Come on, it's not funny!!!
Didn't expect everyone to like it but at least it made me laugh.
1446D2 - Frequency Problem (Hard Version) was my favorite problem in Codeforces. I also loved the solution of Giant Graph from Atcoder.
I really enjoyed solving this problem: 1439C - Greedy Shopping
1421D - Hexagons was pretty cool :))
Nothing comes to my mind, just more and more ad-hocs/"deeply analyze some complicated process" everywhere ;_;
To be serious, here's the list, maybe I'll update it:
some CodeChef problem (by 300iq)
Grand Prix of Zhejiang — D, F, K (by jqdai0815 (I guess))
Grand Prix of Tokyo — B, L (by ???)
Code Jam round 3 — D (by ???)
Recalculating was written by tbuzzelli and Pablo Heiber.
1392E — Omkar and Duck (Global Round 10) was my favourite.
I second this (it was written by coauthor of that round, not me).
1363C - Game On Leaves
AGC50A — AtCoder Jumper: Solution is so elegant and simple, yet so hard to think of :')
1458C — Latin Square: Really cool structure to the problem
https://mirror.codeforces.com/contest/1457/problem/D Usually, I find bitmasks quite tricky, but solving this problem during the contest was fun
Most high-rated users will probably post some hard problems so I'll post an easy one: Dodecagon.
i was expecting a FFT problem
Ironically I don't actually have many favorite FFT problems. The only one that stuck with me is 286E - Ladies' Shop and it is from 2013. Maybe good FFT problems are still too hard for me :P. I don't think I can like the FFT part of the problem unless you have to actually do something interesting in the "frequency domain". But often a FFT problem is just bashing formulas until you get one that FFT can efficiently calculate.
IOI 2020 — Mushrooms: dynamic programming-like approach to find optimal query sequence as described here.
I really liked this problem and I also really enjoyed solving this problem.
I generally don't have favorite problems, but I liked very much the problems from IOI2020!
man I really liked 1392D - Omkar and Bed Wars. In fact, now that I think about that that entire round was very high quality. I really wish those psetters would write more in the future.
This was a cool dp with optimization problem by dario2994
https://mirror.codeforces.com/contest/1427/problem/C
This . I solved using different approach. I still don't know its proof.
102893I - Points and Segments
This problem require you to believe that winner of described game is determineted, and all is you need — make legal turns
1316C - Primitive Primes
1334E - Divisor Paths
1408F - Two Different
1451E2 - Bitwise Queries (Hard Version)
I really loved Comparing Plants from IOI 2020 Day 1, the setup is really natural but involves ideas that were not obvious at all.
The reduction to the directed graph was really interesting and I was surprised how things like 2k decomposition and toposort could come out a seemingly non-graph problem
1427G - One Billion Shades of Grey
btw, if I pretended that Good Bye 2019 happens on 2020, I would like to nominate 1270G - Subset with Zero Sum
1270G - Subset with Zero Sum is realy great, thanks and Happy New Year:)
1451E2 - Bitwise Queries (Hard Version) It's neat and interesting
1325E - Ehab's REAL Number Theory Problem Truly impressive
Comparing to the last year, I feel that the year 2020 wasn't a productive year even for problemsetters, due to the delay of WF and cancellation of many onsite events. But still I can think of many interesting problems. Problems are roughly ordered chronologically.
Also honorable mention goes to Rooted MST, Insects, Hit, Comparing Plants, Range Diameter Sum.
For me IOI 2020 Biscuits was a cool digit dp problem. Working out the ideas on it during contest just felt really satisfying for some reason.
Although not really a problem, I was amused with 102638C - Anime. And some honorable mentions: New Year Frog, Bacteria Control, Rotation Game
IOI2020 — Stations is definitely TOP 1 in the list of problems that I should've solved during the contest, howerver something went wrong :(
I always forget problems after few days of contest .
1334E - Divisor Paths
1333F - Kate and imperfection
1333D - Challenges in school №41
1406D - Three Sequences
1327D - Infinite Path
1305E - Kuroni and the Score Distribution
1363E - Tree Shuffling
1450C2 - Errich-Tac-Toe (Hard Version)
1437F — Emotional Fishermen
1437E — Make It Increasing
1466F — Euclid's nightmare
I like all problems from AGC 043, 044. Also, 045 D. (basically I tend to like all problems from rng_58, maroonrk, sugim48)
1439A2 - Binary Table (Hard Version) Not sure about my favourite problem(there are quite a few I really liked ) but this problem surely tops my worst problem's list :/
Here are some impressive problems:
I set several problems for the local contest this year, and some of them seem to be pretty nice. Here are my two favorite problems:
(both of them have an elegant $$$O(n)$$$ solution :) )
I will go with 1368F - Lamps on a Circle
1333E — Road to 1600 Pretty constructive problem as I love all chess related problems.
April fools 2020 C- And after happily lived ever they The problem I enjoyed solving the most :)
Enjoyed these two 1451E1 - Bitwise Queries (Easy Version) and 1451E2 - Bitwise Queries (Hard Version)
Maybe 1439A2 - Binary Table (Hard Version) :D
No,it is one of the most terrible problem in 2020.
Took me so long to solve it but finally got it :p 1331A - Is it rated?
It should be 1394C - Boboniu and String
Task D (title: "L") from AtCoder Regular Contest 109, in particular this solution by AnandOza
During the contest, I was trying various ways to map the set of "valid arrangements of stones" into a simpler representation, and failed to get anywhere... the solution of mapping it to
their centroid
is the kind of brilliant solution that makes you facepalm in hindsight
Wow, thanks for the shoutout!
definitely 1355 DIV2 E as it helped me reach CM :D
it was fun to hypothesize that the cost function is first increasing and then decreasing and then apply binary search to find the maxima
1331A - Is it rated? (from April Fools Day Contest 2020) XD
https://mirror.codeforces.com/contest/1370/problem/D https://mirror.codeforces.com/contest/1408/problem/E https://mirror.codeforces.com/contest/1407/problem/E https://mirror.codeforces.com/contest/1391/problem/E https://mirror.codeforces.com/contest/1469/problem/E
1370D is my #1 favorite, the rest are in no particular order
https://mirror.codeforces.com/contest/1286/problem/C2 https://mirror.codeforces.com/contest/1307/problem/F https://mirror.codeforces.com/contest/1368/problem/G https://www.codechef.com/problems/CHEFMOD https://mirror.codeforces.com/contest/1383/problem/C https://mirror.codeforces.com/contest/1383/problem/D https://mirror.codeforces.com/contest/1383/problem/E https://www.codechef.com/OCT20A/problems/RANDKNAP https://mirror.codeforces.com/contest/1428/problem/G2 https://mirror.codeforces.com/contest/1408/problem/G https://mirror.codeforces.com/contest/1444/problem/D https://www.codechef.com/LTIME89A/problems/EFLIP https://mirror.codeforces.com/gym/102623/problem/I https://mirror.codeforces.com/contest/1415/problem/F https://mirror.codeforces.com/contest/1305/problem/G https://www.codechef.com/LTIME91A/problems/XORPALIN
Recently I did topcoder srm 795 div problem which enjoyed a lot and was tough too , great editorial
1442C - Graph Transpositions
1406D - Three Sequences
1370F2 - The Hidden Pair (Hard Version)
1395F - Boboniu and String
Top 3 favorite problems for me.
My favorite problems: 1. https://mirror.codeforces.com/contest/1443/problem/F 2. https://mirror.codeforces.com/contest/1427/problem/C 3. https://mirror.codeforces.com/contest/1359/problem/D 4. https://mirror.codeforces.com/contest/1363/problem/D 5. https://mirror.codeforces.com/contest/1400/problem/D 6. https://mirror.codeforces.com/contest/1384/problem/D
(C1, C2) Div 2 — (A1, A2) Div 1 — Binary Table
There would be many others, but these are the ones I remember very well. In random order:
Of course 1372E - Omkar and Last Floor !
It took me 2 hours to work it out completely.
It requires a lot of thoughts to get a really clean and short solution!
Wonderful!
Of course Latin Square and Pocky Game.
By the way, the latter one is the few problems that i remember their names after i solved them.