# 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```

## 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{
}
}
}

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)