I reviewed all the contests I got negative delta in this year. Then, I found something interesting!
Teza Round 1 (Codeforces Round 1015, Div. 1 + Div. 2) I spent 2 hours in 2084E - Blossom(Given an incomplete permutation and count the possible valid permutations) so that I didn't have enough time to solve G1. Rating -= 59.
Codeforces Round 1021 (Div. 1) No counting problems but I spent more than 1 hour in a stupid ExGCD problem. Rating -= 38.
Codeforces Round 1024 (Div. 1) I didn't solve 2101D - Mani and Segments(Given a permutation and count the valid subarrays, which is only *2500) within 1.5 hours. Rating -= 28.
EPIC Institute of Technology Round Summer 2025 (Codeforces Round 1036, Div. 1 + Div. 2) I didn't solve 2124F2 - Appending Permutations (Hard Version)(Given some restrictions, count the valid permutations) as I quickly solved the easy version with 1.5 hours left. Rating -= 2.
Codeforces Round 1040 (Div. 1) I didn't solve 2129D - Permutation Blackhole(Given some restrictions, count the valid permutations), which is obviously a range DP. Rating -= 96.
Atto Round 1 (Codeforces Round 1041, Div. 1 + Div. 2) I didn't solve 2127F - Hamed and AghaBalaSar(Calculate F(a) for all valid array a modulo 1e9+7). I even couldn't come up with any possible observations except transforming the function F and try to enumerate a[n] and how many times it appears(obviously O(nlnn)). Rating -= 43.
I have a conclusion now.
If one contest have a problem like counting valid permutations or given few numbers and calculate something % 1e9+7 / 998244353 in the place of 1D, I can easily get a negative delta while everyone other than me can solve it with ease.
What should I do???
do more counting problems
This is a great advice. I will do that. Thank you!
its crazy that "do more xyz" when you suck at "xyz" is always a great advice no matter how good you are
Buy or download mathematic books on combinatorics and do their exercises.
use your fingers
how can i count numbers above 1024?
use your toes
Try more AtCoder problems.
Excuse me! Can you tell me how linyue's mind works?
Figuring this out is much harder than solving AGC058F :skull:
How it feels giving advice to the person at rank 1 in majority contest.... while myself being an unrated
Not good at counting problems either
but as an ICPC competitor, I leave these problems to my teammates XD
I got stuck on many of those same problems — for me the issue with them is that it's very easy to go down the wrong path and waste too much time on an approach that either doesn't work or does but is an implementation nightmare. I haven't found any solution to this other than to do many problems in the difficulty range to get a feel of how complex a solution should be, and periodically ask myself if I'm overthinking it
can someone suggest books to improve on combinatorics problems?
literally me
Blame authors for creating counting problems