Reverse Pairs

Hard

Given an integer array nums, return the number of reverse pairs. 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

Example 2

Input:  nums = [2,4,3,5,1]
Output: 3

Constraints

• 1 <= nums.length <= 5*10^4
• -2^31 <= nums[i] <= 2^31 - 1