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## Problem

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

Example 1:

Input: nums1 = [1,2], nums2 = [-2,-1], nums3 = [-1,2], nums4 = [0,2]
Output: 2
Explanation:
The two tuples are:
1. (0, 0, 0, 1) -> nums1[0] + nums2[0] + nums3[0] + nums4[1] = 1 + (-2) + (-1) + 2 = 0
2. (1, 1, 0, 0) -> nums1[1] + nums2[1] + nums3[0] + nums4[0] = 2 + (-1) + (-1) + 0 = 0


Example 2:

Input: nums1 = [0], nums2 = [0], nums3 = [0], nums4 = [0]
Output: 1


Constraints:

• n == nums1.length
• n == nums2.length
• n == nums3.length
• n == nums4.length
• 1 <= n <= 200
• -2^28 <= nums1[i], nums2[i], nums3[i], nums4[i] <= 2^28

## Code

class Solution {
public int fourSumCount(int[] nums1, int[] nums2, int[] nums3, int[] nums4) {
int len = nums1.length;
HashMap<Integer, Integer> map = new HashMap<>();

for(int i = 0; i < len; i++){
for(int j = 0; j < len; j++){
int sum = nums1[i] + nums2[j];
map.put(sum, map.getOrDefault(sum, 0) + 1);
}
}

int res = 0;

for(int i = 0; i < len; i++){
for(int j = 0; j < len; j++){
int sum = nums3[i] + nums4[j];
res += map.getOrDefault(-1 * sum, 0);
}
}

return res;
}
}