454. 4Sum II

Given four integer arrays nums1, nums2, nums3, and nums4 all of length n, return the number of tuples (i, j, k, l) such that:

• 0 <= i, j, k, l < n
• nums1[i] + nums2[j] + nums3[k] + nums4[l] == 0

class Solution {
public:
    int fourSumCount(vector<int>& A, vector<int>& B, vector<int>& C, vector<int>& D) {
        int res = 0;
        unordered_map<int, int> tmp1, tmp2, s1;
        for(auto& n: A) tmp1[n]++;
        for(auto& n: B) {
            for(auto& [k, v]: tmp1) {
                s1[k+n]+=v;
            }
        }
        for(auto& n: C) tmp2[n]++;
        for(auto& n: D) {
            for(auto& [k, v]: tmp2) {
                res += s1[-(k+n)] * v;
            }
        }
        
        return res;
    }
};