4Sum II.cpp

class Solution {
public:
   int fourSumCount(vector<int>& A, vector<int>& B, vector<int>& C, vector<int>& D) {
      unordered_map<int, int> map;
      int res = 0;
      for (int i = 0; i < A.size(); ++i)
      {
         for (int j = 0; j < B.size(); ++j)
         {
            int sum = A[i] + B[j];
            ++map[sum];
         }
      }
      for (int i = 0; i < C.size(); ++i)
      {
         for (int j = 0; j < D.size(); ++j)
         {
            int sum = C[i] + D[j];
            res += map[-sum];
         }
      }
      return res;
   }
};