# 1995. Count Special Quadruplets

<https://leetcode.com/problems/count-special-quadruplets>

## Description

Given a **0-indexed** integer array `nums`, return *the number of **distinct** quadruplets* `(a, b, c, d)` *such that:*

* `nums[a] + nums[b] + nums[c] == nums[d]`, and
* `a < b < c < d`

**Example 1:**

```
**Input:** nums = [1,2,3,6]
**Output:** 1
**Explanation:** The only quadruplet that satisfies the requirement is (0, 1, 2, 3) because 1 + 2 + 3 == 6.
```

**Example 2:**

```
**Input:** nums = [3,3,6,4,5]
**Output:** 0
**Explanation:** There are no such quadruplets in [3,3,6,4,5].
```

**Example 3:**

```
**Input:** nums = [1,1,1,3,5]
**Output:** 4
**Explanation:** The 4 quadruplets that satisfy the requirement are:
- (0, 1, 2, 3): 1 + 1 + 1 == 3
- (0, 1, 3, 4): 1 + 1 + 3 == 5
- (0, 2, 3, 4): 1 + 1 + 3 == 5
- (1, 2, 3, 4): 1 + 1 + 3 == 5
```

**Constraints:**

* `4 <= nums.length <= 50`
* `1 <= nums[i] <= 100`

## ac

```java
```