The more problems you work out, the more emptyness you will fill. Maybe it's better to think of brushing problems as a mind challenge. And enjoy the process.
Normal sorting complexity 1D 2D binary index tree KMP Quickselect complexity, quicksort Multirandom selection https://discuss.leetcode.com/topic/53753/brief-explanation-for-reservoir-sampling
DP on trees... http://codeforces.com/blog/entry/20935