antontrygubO_o's blog

What were your favorite problems in 2020?
By antontrygubO_o, history, 4 years ago, In English

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?

  • Vote: I like it
  • +421
  • Vote: I do not like it

Comments Comments (75)
| Write comment?
»
4 years ago, # |
  Vote: I like it +5 Vote: I do not like it

I found this problem to be cool: 1463E - План лекций. It took me around 2 hours to understand the solution and another 4 to implement, but totally worth my time!

→ Reply
»
4 years ago, # |
  Vote: I like it +68 Vote: I do not like it

1338E - JYPnation

→ Reply
»
4 years ago, # |
  Vote: I like it +27 Vote: I do not like it

E. Ski Accidents (Codeforces Global Round 8)

→ Reply
»
4 years ago, # |
  Vote: I like it -14 Vote: I do not like it

This problem .. I was so proud solving it during contest

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

1331E - Смайлик Жордана

→ Reply
»
4 years ago, # |
  Vote: I like it +87 Vote: I do not like it

1458D - Флипы и развороты

→ Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it +21 Vote: I do not like it

I liked Monogon's problems.

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

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.

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

I always forget what problems I solved, so here are some good ones from my recent solves:

https://codeforces.net/contest/1406/problem/D

https://codeforces.net/contest/1451/problem/E2

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

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.

→ Reply
»
4 years ago, # |
  Vote: I like it -79 Vote: I do not like it

4A - Арбуз :D

→ Reply
»
4 years ago, # |
  Vote: I like it +83 Vote: I do not like it

1446D2 - Задача о частотах (сложная версия) was my favorite problem in Codeforces. I also loved the solution of Giant Graph from Atcoder.

→ Reply
»
4 years ago, # |
  Vote: I like it -16 Vote: I do not like it

I really enjoyed solving this problem: 1439C - Жадный шоппинг

→ Reply
»
4 years ago, # |
  Vote: I like it -29 Vote: I do not like it

1421D - Шестиугольники was pretty cool :))

→ Reply
»
4 years ago, # |
← Rev. 5   Vote: I like it +63 Vote: I do not like it

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 ???)

→ Reply
»
4 years ago, # |
  Vote: I like it +35 Vote: I do not like it

1392E — Omkar and Duck (Global Round 10) was my favourite.

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +2 Vote: I do not like it

    I second this (it was written by coauthor of that round, not me).

    → Reply
»
4 years ago, # |
  Vote: I like it +16 Vote: I do not like it

1363C - Игра на листьях

→ Reply
»
4 years ago, # |
  Vote: I like it +31 Vote: I do not like it

AGC50A — AtCoder Jumper: Solution is so elegant and simple, yet so hard to think of :')

1458C — Latin Square: Really cool structure to the problem

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

https://codeforces.net/contest/1457/problem/D Usually, I find bitmasks quite tricky, but solving this problem during the contest was fun

→ Reply
»
4 years ago, # |
  Vote: I like it +26 Vote: I do not like it

Most high-rated users will probably post some hard problems so I'll post an easy one: Dodecagon.

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +7 Vote: I do not like it

    i was expecting a FFT problem

    → Reply
    • »
      »
      »
      4 years ago, # ^ |
        Vote: I like it +18 Vote: I do not like it

      Ironically I don't actually have many favorite FFT problems. The only one that stuck with me is 286E - Дамский магазин 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.

      → Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

IOI 2020 — Mushrooms: dynamic programming-like approach to find optimal query sequence as described here.

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I really liked this problem and I also really enjoyed solving this problem.

→ Reply
»
4 years ago, # |
  Vote: I like it +8 Vote: I do not like it

I generally don't have favorite problems, but I liked very much the problems from IOI2020!

→ Reply
»
4 years ago, # |
  Vote: I like it +15 Vote: I do not like it

man I really liked 1392D - Омкар и Война кроватей. 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.

→ Reply
»
4 years ago, # |
  Vote: I like it +38 Vote: I do not like it

This was a cool dp with optimization problem by dario2994
https://codeforces.net/contest/1427/problem/C

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

This . I solved using different approach. I still don't know its proof.

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

102893I - Точки и отрезки

Spoiler to problem!
→ Reply
»
4 years ago, # |
  Vote: I like it +10 Vote: I do not like it

1316C - Простые простые

1334E - Пути по делителям

1408F - Два различных

1451E2 - Битовые запросы (усложенная версия)

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

I really loved Comparing Plants from IOI 2020 Day 1, the setup is really natural but involves ideas that were not obvious at all.

Spoiler
→ Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it +41 Vote: I do not like it

1427G - Миллиард оттенков серого

btw, if I pretended that Good Bye 2019 happens on 2020, I would like to nominate 1270G - Подмножество с суммой ноль

→ Reply
»
4 years ago, # |
← Rev. 3   Vote: I like it +15 Vote: I do not like it

1451E2 - Битовые запросы (усложенная версия) It's neat and interesting

1325E - НАСТОЯЩАЯ Теория чисел от Ехаба Truly impressive

→ Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it +68 Vote: I do not like it

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.

→ Reply
»
4 years ago, # |
  Vote: I like it +30 Vote: I do not like it
→ Reply
»
4 years ago, # |
  Vote: I like it +8 Vote: I do not like it

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.

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

Although not really a problem, I was amused with 102638C - Аниме. And some honorable mentions: New Year Frog, Bacteria Control, Rotation Game

→ Reply
  • »
    »
    4 years ago, # ^ |
      Vote: I like it +15 Vote: I do not like it

    IOI2020 — Stations is definitely TOP 1 in the list of problems that I should've solved during the contest, howerver something went wrong :(

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

I always forget problems after few days of contest .

→ Reply
»
4 years ago, # |
← Rev. 3   Vote: I like it +10 Vote: I do not like it

1334E - Пути по делителям
1333F - Катя и несовершенства
1333D - Досуг в школе №41
1406D - Три последовательности
1327D - Бесконечный путь
1305E - Курони и разбалловка
1363E - Перестановка дерева
1450C2 - Крестики Нолики Errichto (сложная версия)

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

1437F — Emotional Fishermen
1437E — Make It Increasing
1466F — Euclid's nightmare

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

I like all problems from AGC 043, 044. Also, 045 D. (basically I tend to like all problems from rng_58, maroonrk, sugim48)

→ Reply
»
4 years ago, # |
  Vote: I like it +47 Vote: I do not like it

1439A2 - Бинарная таблица (сложная версия) Not sure about my favourite problem(there are quite a few I really liked ) but this problem surely tops my worst problem's list :/

→ Reply
»
4 years ago, # |
  Vote: I like it +13 Vote: I do not like it

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 :) )

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I will go with 1368F - Лампочки по кругу

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

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 :)

→ Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it +10 Vote: I do not like it

Enjoyed these two 1451E1 - Битовые запросы (упрощенная версия) and 1451E2 - Битовые запросы (усложенная версия)

→ Reply
»
4 years ago, # |
  Vote: I like it +10 Vote: I do not like it
→ Reply
»
4 years ago, # |
  Vote: I like it -15 Vote: I do not like it

Maybe 1439A2 - Бинарная таблица (сложная версия) :D

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

Took me so long to solve it but finally got it :p 1331A - Is it rated?

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

It should be 1394C - Boboniu и строка

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

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

Spoiler

is the kind of brilliant solution that makes you facepalm in hindsight

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

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

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

1331A - Is it rated? (from April Fools Day Contest 2020) XD

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

https://codeforces.net/contest/1370/problem/D https://codeforces.net/contest/1408/problem/E https://codeforces.net/contest/1407/problem/E https://codeforces.net/contest/1391/problem/E https://codeforces.net/contest/1469/problem/E

1370D is my #1 favorite, the rest are in no particular order

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

https://codeforces.net/contest/1286/problem/C2 https://codeforces.net/contest/1307/problem/F https://codeforces.net/contest/1368/problem/G https://www.codechef.com/problems/CHEFMOD https://codeforces.net/contest/1383/problem/C https://codeforces.net/contest/1383/problem/D https://codeforces.net/contest/1383/problem/E https://www.codechef.com/OCT20A/problems/RANDKNAP https://codeforces.net/contest/1428/problem/G2 https://codeforces.net/contest/1408/problem/G https://codeforces.net/contest/1444/problem/D https://www.codechef.com/LTIME89A/problems/EFLIP https://codeforces.net/gym/102623/problem/I https://codeforces.net/contest/1415/problem/F https://codeforces.net/contest/1305/problem/G https://www.codechef.com/LTIME91A/problems/XORPALIN

→ Reply
»
4 years ago, # |
  Vote: I like it -8 Vote: I do not like it

Recently I did topcoder srm 795 div problem which enjoyed a lot and was tough too , great editorial

→ Reply
»
4 years ago, # |
  Vote: I like it +10 Vote: I do not like it
→ Reply
»
4 years ago, # |
  Vote: I like it +35 Vote: I do not like it

Top 3 favorite problems for me.

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

My favorite problems: 1. https://codeforces.net/contest/1443/problem/F 2. https://codeforces.net/contest/1427/problem/C 3. https://codeforces.net/contest/1359/problem/D 4. https://codeforces.net/contest/1363/problem/D 5. https://codeforces.net/contest/1400/problem/D 6. https://codeforces.net/contest/1384/problem/D

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

(C1, C2) Div 2 — (A1, A2) Div 1 — Binary Table

→ Reply
»
4 years ago, # |
← Rev. 2   Vote: I like it +28 Vote: I do not like it

There would be many others, but these are the ones I remember very well. In random order:

→ Reply
»
4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

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!

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

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.

→ Reply