454. 四数相加 II

class Solution {
public:
    int fourSumCount(vector<int>& A, vector<int>& B, vector<int>& C, vector<int>& D) {
        unordered_map<int, int> hash_AB;
        int a, b, c, d, count = 0, sum;
        for(const auto &a:A)
        {
            for(const auto&b:B)
            {
                hash_AB[a+b]++;
            }
        }
        for(const auto &c:C)
        {
            for(const auto &d:D)
            {
                sum = -(c+d);
                if(hash_AB[sum]!=0)
                {
                    count += hash_AB[sum];
                }
            }
        }
        return count;
    }
};