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

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