256. Reverse Pairs

HardSorting

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].

Examples

Input: [1,3,2,3,1]

Output: 2

Explanation: Reverse pairs: (3,1) at indices (1,4) and (3,4).

Constraints

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