Random numbers in leetcode 710 blacklist [random numbers] the leetcode path of heroding

Their thinking ：
Realization [0,n-m] Interval blacklist pair [n-m, n] Non blacklist mapping of , And then take [0,n-m] random number , If it is a white list, return directly , Otherwise, the mapped white name singular number is returned , The code is as follows ：

class Solution {
    
    unordered_map<int, int> b2w;
    int bound;

public:
    Solution(int n, vector<int> &blacklist) {
    
        int m = blacklist.size();
        bound = n - m;
        unordered_set<int> 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;
    }
};