数组中的逆序对

题目描述

在数组中的两个数字，如果前面一个数字大于后面的数字，则这两个数字组成一个逆序对。输入一个数组,求出这个数组中的逆序对的总数P

 

使用归并排序的思路

 

public class Solution {

    public int InversePairs(int [] array) {

        if(array == null || array.length == 0) {

            return 0;

        }

        int[] copy = new int[array.length];       

        int count = InversePairsCore(array,copy,0,array.length-1);//数值过大求余

        return count;

    }

    private int InversePairsCore(int[] array,int[] copy,int low,int high) {

        if(low == high) {

            return 0;

        }

        int mid = (low + high) >> 1;

        int leftCount = InversePairsCore(array, copy, low, mid);

        int rightCount = InversePairsCore(array, copy, mid + 1, high);

        int count = 0;

        int i = mid;

        int j = high;

        int locCopy = high;

        while(i >= low && j > mid) {

            if(array[i] > array[j]) {

                count += j-mid;

                copy[locCopy--] = array[i--];

            }

            else {

                copy[locCopy--] = array[j--];

            }

        }

        while (i >= low) {

            copy[locCopy--] = array[i--];

        }

        while (j > mid) {

            copy[locCopy--] = array[j--];

        }

        for(int s = low; s <= high; s++) {

            array[s] = copy[s];

        }

        return leftCount + rightCount + count;

    }

}