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g2501_2600.s2552_count_increasing_quadruplets.readme.md Maven / Gradle / Ivy

2552\. Count Increasing Quadruplets

Hard

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.