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