710

class Solution {
public:
    int wlen;
    unordered_map<int, int> m;

    Solution(int n, vector<int>& blacklist) {
        wlen = n-blacklist.size();
        unordered_set<int> white;
        for(int i=wlen; i<n; i++) white.insert(i);
        for(int bnum : blacklist) white.erase(bnum);
        auto wi = white.begin();
        for(int bnum: blacklist){
            if(bnum < wlen)
                m[bnum] = *wi++;
        }
    }

    int pick() {
        int k = rand() % wlen;
        return m.count(k) ? m[k] : k;
    }
};

/**
 * Your Solution object will be instantiated and called as such:
 * Solution* obj = new Solution(n, blacklist);
 * int param_1 = obj->pick();
 */