class Solution {
public:
    int fourSumCount(vector<int>& A, vector<int>& B, vector<int>& C, vector<int>& D) {
        unordered_map <int, int> mp; // 哈希表：A中一个元素加B中一个元素的和的计数
        int cnt = 0;
        for(int i = 0; i < A.size(); i++) {
            for(int j = 0; j < B.size(); j++) {
                int sum = A[i] + B[j];
                mp[sum]++;
            }
        }

        for(int i = 0; i < C.size(); i++) {
            for(int j = 0; j < D.size(); j++) {
                int sum = C[i] + D[j];
                if(mp.find(-sum) != mp.end())
                    cnt += mp[-sum];
            }
        }
        return cnt;
    }
};