#1995

Count Special Quadruplets

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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

Solution