LeetCode 710 黑名单中的随机数[随机数] HERODING的LeetCode之路

解题思路：
实现[0,n-m]区间黑名单对[n-m, n]的非黑名单映射，然后取[0,n-m]随机数，如果是白名单直接返回，否则返回映射的白名单数，代码如下：

class Solution {
    unordered_map b2w;
    int bound;

public:
    Solution(int n, vector &blacklist) {
        int m = blacklist.size();
        bound = n - m;
        unordered_set black;
        for (int b: blacklist) {
            if (b >= bound) {
                black.emplace(b);
            }
        }

        int w = bound;
        for (int b: blacklist) {
            if (b < bound) {
                while (black.count(w)) {
                    ++w;
                }
                b2w[b] = w++;
            }
        }
    }

    int pick() {
        int x = rand() % bound;
        return b2w.count(x) ? b2w[x] : x;
    }
};