leetcode-454

class Solution {
public:
    int fourSumCount(vector<int>& nums1, vector<int>& nums2, vector<int>& nums3, vector<int>& nums4) {
        unordered_map<int,int> result_map;
        for (int a:nums1){
            for(int b:nums2){
                result_map[a+b]++;
            }
        }
        int count = 0;
        for (int  c:nums3){
            for(int d:nums4){
                if(result_map.find(0-(c+d))!=result_map.end())
                {
                    count+=result_map[0-(c+d)];
                }
            }
        }
        return count;
    }
};