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

Note:

1. The length of the given array will not exceed 50,000.
2. All the numbers in the input array are in the range of 32-bit integer.

class Solution {
    public int reversePairs(int[] nums) {
        return help(nums, 0, nums.length - 1);
    }
    
    public int help(int[] nums, int lo, int hi) {
        if(lo >= hi) return 0;
        int mid = lo + (hi - lo) / 2;
        int res = help(nums, lo, mid) + help(nums, mid + 1, hi);
        Arrays.sort(nums, lo, mid + 1);
        Arrays.sort(nums, mid + 1, hi + 1);
        int i = lo;
        int j = mid + 1;
        while(i <= mid && j <= hi) {
            if((long) nums[i] > (long) nums[j] * 2) {
                res += mid - i + 1;
                j++;
            }
            else i++;            
        }
        return res;
    }
}

https://www.youtube.com/watch?v=j68OXAMlTM4