#493
Reverse Pairs
hard · verified · 33.8% accepted · 6,974 likes · top 11%
array · binary search · divide and conquer · binary indexed tree · segment tree · merge sort · ordered set
Description
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) --> 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
Solution