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# 0454. 4Sum II

<https://leetcode.com/problems/4sum-ii>

## Description

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`
* `-228 <= nums1[i], nums2[i], nums3[i], nums4[i] <= 228`

## ac

```java
class Solution {
    public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
        // edge cases
        if (A.length == 0) return 0;

        // 2 maps
        Map<Integer, Integer> map1 = new HashMap<Integer, Integer>();
        Map<Integer, Integer> map2 = new HashMap<Integer, Integer>();
        int n = A.length;

        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                int k1 = A[i] + B[j];
                int k2 = C[i] + D[j];
                map1.put(k1, map1.getOrDefault(k1, 0) + 1);
                map2.put(k2, map2.getOrDefault(k2, 0) + 1);
            }
        }

        // iterate 2 maps
        int res = 0;
        for (int k : map1.keySet()) {
            if (map2.containsKey(-k)) {  // find tuples
                res += map1.get(k) * map2.get(-k);
            }
        }

        return res;
    }
}
```

actually one map is enough.

```java
class Solution {
    public int fourSumCount(int[] A, int[] B, int[] C, int[] D) {
        // edge cases
        if (A.length == 0) return 0;

        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        int n = A.length;

        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                int k = A[i] + B[j];
                map.put(k, map.getOrDefault(k, 0) + 1);
            }
        }

        int res = 0;
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                int k = C[i] + D[j];
                if (map.containsKey(-k)) {  // find tuples
                    res += map.get(-k);
                }
            }
        }

        return res;
    }
}
```