problem D from CodeTON Round 8, my code get MLE: https://codeforces.com/contest/1942/submission/254227380, but why? I solve it in O(nklogn), and memory for O(nk), and MLE on test 5.

Anyone has some advice for practice? Sometimes I can solve difficulty up to 2400 but sometimes can't.

upd: Can anyone suggest some difficult and important algorithm that a GM needs to learn?