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 -2^{28} to 2^{28} – 1 and the result is guaranteed to be at most 2^{31} – 1.

**Example:**

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

## Solution

### C# Program

public class Solution { public int FourSumCount(int[] A, int[] B, int[] C, int[] D) { var map = new Dictionary<int,int>(); int result = 0; for(int i=0;i<C.Length;i++) { for(int j=0;j<D.Length;j++) { int sum = C[i] + D[j]; if(map.ContainsKey(sum)) { map[sum]++; } else{ map.Add(sum,1); } } } for(int i=0;i<A.Length;i++) { for(int j=0;j<B.Length;j++) { int sum = A[i] + B[j]; if(map.ContainsKey(-1 * sum)) { result += map[-1 * sum]; } } } return result; } }

Time Complexity: O(m * n)

Space Complexity: O(m * n)