2019 was a semiprime year with great contests and some notorious coincidences.

This year I setted some tie-break problems for CodeChef Long Challenge, probably the most interesting and with real life applications was Bacteria Synthesis about generating proteins from a genome using reversals. I also liked Coloring Triangulations which is about moving from one triangulation to another by flipping edges.

From the exact problems I liked LOVEMUFFIN which is a nice ad hoc game, and Pretty Boxes a flow that can be simulated with segment trees.

I also saw many interesting simple problems like Hit the Coconuts a puzzle that can be solved with a dp trick, and my brute forces Ada Rooks.

Why all your favorite problems are from Codechef?

:) I guess because I was more involved in those problems.

Or you have a pretty bad taste ;P

Problem from the previous contest :)

Maybe I forgot about some problems (so this comment may change), but here are some problems I enjoyed a lot:

Codeforces:Dirty Deeds Done Dirt Cheap by Egor.Lifar

Increasing by Modulo by 300iq

Petya and Construction Set by voidmax

To Make 1 by Endagorion

Not Same by nocriz, RDDCCD

AtCoder:Removing Coins by yutaka1999

Negative Cycle by maroonrk

01 Matrix by maroonrk

Neither AB nor BA by maroonrk

In no particular order:

Codeforces:1142C — U2

1178E — Archaeology

1208F — Bits And Pieces

1254C — Point Ordering

Honorable mention: My first Div1E problem :)

OpenCup:(not public)GP of Baltic Sea problem A

GP of Poland problem L (also available as AMPPZ 2019)

GP of Moscow problem M

Other ICPC problems:Jakarta Regional problem D

CCPC Qinhuangdao problem K

KAIST RUN Spring problem D

KAIST RUN Spring problem H

NAIPC problem G

Taipei Regional problem F

Honorable mentions: Many workshop problems I cannot share

Taipei Regional problem F = Codeforces 1103D

Tbh for CF, I prefer your "honorable mention" problem more than the others.

USACO 2019 December Contest, Silver Problem 2. Meetings

G. (Zero XOR Subset)-less

F. Ivan and Burgers

E. The Child and Binary Tree (Old classic problem. It did lead to a beautiful discussion about polynomials among my teammates.)

These are not so hard mainly Div-2 C/D.

1266D - Decreasing Debts by majk

1172C2 - Nauuo and Pictures (hard version) by ouuan

1230C - Anadi and Domino by Radewoosh

https://community.topcoder.com/stat?c=problem_statement&pm=15791 by Errichto

Most of AGC B's and C's.

My taste is constructive problems.

Most satisfying problem for me in 2019.

Finally I found some application of the god-knows-what-the-fk-um-doing-high-school-trigonometry-problems. My life has never been more delightful. Now I can finally jump i̶n̶t̶o̶ through the pole-to-pole connecting earth tunnel (fyi, earth is not flat). Please find my will in directory-

`~/Downloads/.fapfolder/`

It turns out that I have a long list of favorite problems...

JOI Open Contest Jump

IOI Split Line

Romanian Masters of Informatics Day 1 Q1 which I can't find statement...

Some chinese tasks LOJ3046 LOJ3077 LOJ3082 UR18C

Codeforces 1097E 1149C 1158E 1168E 1188D 1204D2 1229E2 1242D 1246E 1268E

Atcoder WTF19C2 WTF19D WTF19E AGC030E AGC035E AGC036D AGC036E AGC040E

300iq contest 2 F H

(some problems of Opencup that i cannot find link)

Hi, this is RMI Day 1 Q1's statement.

In no particular order

Best of 2019)I think GP of Xian is interesting, but I don't know the solution well enough to judge.

For me, a lot of AGC E/F problems.

In order:

CF1270G

CF1142C

CF1174E

CF1205B

CF1110E