class Solution {public:    int fourSumCount(vector<int>& A, vector<int>& B, vector<int>& C, vector<int>& D) {        int n = A.size(); if(!n) return 0;                int ret = 0;        unordered_map<int, unsigned> hm1, hm2;        for(int i = 0; i < n; i ++)        for(int j = 0; j < n; j ++)            hm1[A[i] + B[j]] ++, hm2[C[i] + D[j]] ++;                for(auto &kv: hm1)            if(hm2.find(-kv.first) != hm2.end())                ret += kv.second * hm2[-kv.first];                return ret;    }};