493. Reverse Pairs

Given an array nums, we call (i, j) an important reverse pair if i < j and nums[i] > 2*nums[j].
You need to return the number of important reverse pairs in the given array.

Example1:

Input: [1,3,2,3,1]
Output: 2
Example2:

Input: [2,4,3,5,1]
Output: 3

https://leetcode.com/problems/reverse-pairs/discuss/97280

Solution：

思路：
Time Complexity: O(N) Space Complexity: O(N)

Solution Code：