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Kotlin-based LeetCode algorithm problem solutions, regularly updated
493\. Reverse Pairs
Hard
Given an integer array `nums`, return _the number of **reverse pairs** in the array_.
A **reverse pair** is a pair `(i, j)` where:
* `0 <= i < j < nums.length` and
* `nums[i] > 2 * nums[j]`.
**Example 1:**
**Input:** nums = [1,3,2,3,1]
**Output:** 2
**Explanation:** The reverse pairs are:
(1, 4) --> nums[1] = 3, nums[4] = 1, 3 > 2 \* 1
(3, 4) --> nums[3] = 3, nums[4] = 1, 3 > 2 \* 1
**Example 2:**
**Input:** nums = [2,4,3,5,1]
**Output:** 3
**Explanation:** The reverse pairs are:
(1, 4) --> nums[1] = 4, nums[4] = 1, 4 > 2 \* 1
(2, 4) --> nums[2] = 3, nums[4] = 1, 3 > 2 \* 1
(3, 4) --> nums[3] = 5, nums[4] = 1, 5 > 2 \* 1
**Constraints:**
* 1 <= nums.length <= 5 * 104
* -231 <= nums[i] <= 231 - 1