LeetCode 781. Rabbits in Forest

class Solution {
public:
    int numRabbits(vector<int>& answers) {
        unordered_map<int, int> m;
        for (int y : answers)
            ++m[y];
        int ans = 0;
        for (auto &[y, x] : m)
            ans += (x + y) / (y + 1) * (y + 1);
        return ans;
    }
};