README.md

1995. Count Special Quadruplets

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

Solutions (Python)

1. Solution

class Solution:
    def countQuadruplets(self, nums: List[int]) -> int:
        n = len(nums)
        ret = 0

        for a in range(n):
            for b in range(a + 1, n):
                for c in range(b + 1, n):
                    for d in range(c + 1, n):
                        if nums[a] + nums[b] + nums[c] == nums[d]:
                            ret += 1

        return ret