leetcode

0454.cpp (607B)

 class Solution {
   public:
     int fourSumCount(const vector<int> &nums1, const vector<int> &nums2, const vector<int> &nums3,
                      const vector<int> &nums4) const {
         const int n = size(nums1);
         unordered_map<int, int> um;
         int res = 0;
 
         for (int i = 0; i < n; i++) {
             for (int j = 0; j < n; j++) {
                 um[nums1[i] + nums2[j]]++;
             }
         }
 
         for (int i = 0; i < n; i++) {
             for (int j = 0; j < n; j++) {
                 res += um[-(nums3[i] + nums4[j])];
             }
         }
 
         return res;
     }
 };