Reverse Pairs

For an array A, if i < j, and A [i] > A [j], called (A [i], A [j]) is a reverse pair.
return total of reverse pairs in A.

 

Example

Given A = [2, 4, 1, 3, 5] , (2, 1), (4, 1), (4, 3) are reverse pairs. return 3

 

利用mergesort 达到nlogn的复杂度

public class Solution {
    /**
     * @param A an array
     * @return total of reverse pairs
     */
    int count = 0;
    int[] helper;
    public long reversePairs(int[] A) {
        // Write your code here
        if(A == null || A.length == 0) return 0;
        helper = new int[A.length];
        mergeCount(A, 0, A.length-1);
        return count;
    }

    private void mergeCount(int[] A, int low, int high) {
        if (low==high) {
            return;
        }
        int mid = (low+high)/2;
        mergeCount(A, low, mid);
        mergeCount(A, mid+1, high);
        merge(A, low, mid, high);
    }
    
    private void merge(int[] A, int low, int mid, int high){
        for(int i=low; i<=high; i++){
            helper[i] = A[i];
        }
        int i=low;
        int j= mid+1;
        int k=low;
        
        while(i<=mid&&j<=high){
            if(helper[i]>helper[j]){
                A[k] = helper[j];
                j++;
                count += (mid-i+1);
            }else{
                A[k] = helper[i];
                i++;
            }
            k++;
        }
        while(i<=mid){
            A[k++] = helper[i++];
        }
    }
}