Count Increasing Quadruplets - Solution & Explanation

Given a 0-indexed integer array nums of size n containing all numbers from 1 to n, return the number of increasing quadruplets.

A quadruplet (i, j, k, l) is increasing if:

• 0 <= i < j < k < l < n, and
• nums[i] < nums[k] < nums[j] < nums[l].

 

Example 1:

Input: nums = [1,3,2,4,5]
Output: 2
Explanation: 
- When i = 0, j = 1, k = 2, and l = 3, nums[i] < nums[k] < nums[j] < nums[l].
- When i = 0, j = 1, k = 2, and l = 4, nums[i] < nums[k] < nums[j] < nums[l]. 
There are no other quadruplets, so we return 2.

Example 2:

Input: nums = [1,2,3,4]
Output: 0
Explanation: There exists only one quadruplet with i = 0, j = 1, k = 2, l = 3, but since nums[j] < nums[k], we return 0.

 

Constraints:

• 4 <= nums.length <= 4000
• 1 <= nums[i] <= nums.length
• All the integers of nums are unique. nums is a permutation.