归并排序求解逆序对

打印所有的逆序对，并求出逆序对的总数。

package class02P;

import java.util.Arrays;

public class Code02P_ReversePair {
    public static int ReversePair(int[] arr){
        if(arr == null && arr.length < 2)  return 0;
        return MergeSort(arr, 0, arr.length - 1);
    }
    public static int MergeSort(int[] arr, int l, int r){
        if(l == r) return 0;
        int mid = l +((r - l) >> 1);
        return MergeSort(arr, l, mid )
                + MergeSort(arr,mid + 1, r)
                + Merge(arr, l, mid, r);
    }

    public static int Merge(int[] arr, int l, int m, int r){
        int[] help = new int[r - l + 1];
        int p1 = l, p2 = m + 1;
        int i = 0;
        int result = 0;
        while(p1 <= m && p2 <= r){
            if(arr[p1] <= arr[p2]){
                help[i ++] = arr[p2 ++];
            }
            else{
                result ++;
                System.out.println("逆序对：" + arr[p1] + " " + arr[p2]);
                help[i ++] = arr[p2 ++];
            }
        }

        if(m - p1 > 0){
            result += (m - p1) * (r - m);
            for(int j = p1 + 1; j <= m; j ++){
                for (int k = m + 1; k <= r; k++) {
                    System.out.println("逆序对：" + arr[j] + " " + arr[k]);
                }
            }
        }

        while(p1 <= m){
            help[i ++] = arr[p1 ++];
        }
        while(p2 <= r){
            help[i ++] = arr[p2 ++];
        }
        for (i = 0; i < help.length; i++) {
            arr[l + i] = help[i];
        }

        return result;
    }

    public static void main(String[] args) {
        int[] arr = new int[5];
        for (int i = 0; i < 5; i++) {
            arr[i] = 5 - i;
        }
        int a = ReversePair(arr);
        System.out.println(a);
    }

}