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> umap;
        for(int a: A){
            for(int b: B){
                umap[a + b]++;
            }
        }
        int count = 0;
        for(int c: C){
            for(int d: D){
                if(umap.find(0 - (c + d)) != umap.end()){
                    count += umap[0 - (c + d)];
                }
            }
        }
        return count;
    }
};