Leetcode 710. Random Pick with Blacklist

Given n and a blacklist of unique integers, design a data structure that returns a uniformly random integer from [0, n-1] excluding the blacklist while minimizing calls to the built-in random generator. Preprocess efficiently (using about O(blacklist.size) extra space) so picks can be done in O(1) time by compacting the valid values and remapping sampled indices.