题目：

题解：

class Solution {
public:int fourSumCount(vector<int>& A, vector<int>& B, vector<int>& C, vector<int>& D) {unordered_map<int, int> countAB;for (int u: A) {for (int v: B) {++countAB[u + v];}}int ans = 0;for (int u: C) {for (int v: D) {if (countAB.count(-u - v)) {ans += countAB[-u - v];}}}return ans;}
};