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Reverse Pairs

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

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

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

Reverse Pairs diagram

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 * 10^4
  • -231 <= nums[i] <= 231 - 1

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