454. 4Sum II

Given four lists A, B, C, D of integer values, compute how many tuples (i, j, k, l) there are such that A[i] + B[j] + C[k] + D[l] is zero.

To make problem a bit easier, all A, B, C, D have same length of N where 0 ≤ N ≤ 500. All integers are in the range of -228 to 228 - 1 and the result is guaranteed to be at most 231 - 1.

Example:

Input:
A = [ 1, 2]
B = [-2,-1]
C = [-1, 2]
D = [ 0, 2]

Output:
2

Explanation:
The two tuples are:
1. (0, 0, 0, 1) -> A[0] + B[0] + C[0] + D[1] = 1 + (-2) + (-1) + 2 = 0
2. (1, 1, 0, 0) -> A[1] + B[1] + C[0] + D[0] = 2 + (-1) + (-1) + 0 = 0

// Hashmap
int fourSumCount(vector<int>& A, vector<int>& B, vector<int>& C, vector<int>& D) { // time: O(n^2); space: O(n^2)
    int res = 0;
    unordered_map<int, int> mp;
    for (int a : A) {
        for (int b : B) {
            ++mp[a + b];
        }
    }
    for (int c : C) {
        for (int d : D) {
            int target = -1 * (c + d);
            res += mp[target];
        }
    }
    return res;
}