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) --> nums1 = 3, nums4 = 1, 3 > 2 \* 1

(3, 4) --> nums3 = 3, nums4 = 1, 3 > 2 \* 1

Example 2:

Input: nums = 2,4,3,5,1

Output: 3

Explanation: The reverse pairs are:

(1, 4) --> nums1 = 4, nums4 = 1, 4 > 2 \* 1

(2, 4) --> nums2 = 3, nums4 = 1, 3 > 2 \* 1

(3, 4) --> nums3 = 5, nums4 = 1, 5 > 2 \* 1

Constraints:

• <code>1 <= nums.length <= 5 * 10⁴</code>

• <code>-2³¹<= numsi<= 2³¹ - 1</code>