LeetCode [493] Reverse Pairs

 493. Reverse Pairs

Hard

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) {
        int n = nums.length;
        return helper(nums, 0, n-1);
    }

    int helper(int[] nums, int start, int end){
        if(end-start+1<2) return 0;
        int mid = (start+end)/2;
        int ret = helper(nums, start, mid);
        ret += helper(nums, mid+1, end);

        int j = mid+1, k = mid+1, t = 0;
        int[] cache = new int[end-start+1];
        for(int i=start; i<=mid; ++i){
            while(j<=end && (long)2*nums[j]<(long)nums[i]) j++;
            ret += j-mid-1;
            while(k<=end && nums[k]<=nums[i]) cache[t++] = nums[k++];
            cache[t++] = nums[i];
        }
        System.arraycopy(cache, 0, nums, start, t);
        return ret;
    }
}