Problem Statement

Given an array of integers nums, return the count of reverse pairs in the array.

A reverse pairis a pair (i, j) where:

• 0 <= i < j < nums.length and
• nums[i] > 2 * nums[j].

Examples

• Example 1:

  • Input: nums = [4, 3, 4, 1]
  • Expected Output: 3
  • Justification: The three reverse pairs are (0, 3), (1, 3) and (2, 3).

• Example 2:

  • Input: nums = [4, 1, 2]
  • Expected Output: 1
  • Justification: There is only one reverse pair (0, 1) since 4 > 2 * 1.

• Example 3:

  • Input: nums = [6,5,4,3,2,1]
  • Expected Output: 6
  • Justification: The siz reverse pairs are (0, 4), (0, 5),(1, 4), (1, 5), (2, 5) and (3, 5).

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