利用merge sort寻找逆序对

 1     public class InversionCount {
 2         public static void main(String[] args){
 3             int[] arr = new int[]{2,30,19,6,7,3,5};
 4             System.out.println("before sort：");
 5             Sort.printarr(arr);
 6             int num = countinversion(arr,0,arr.length-1);
 7             System.out.println("the number of arr‘s inversions :"+num);
 8         }
 9     
10         private static int countinversion(int[] arr,int start,int end) {
11             int count = 0;
12             if(start<end){
13                 int divide = (end+start)/2;
14                 count+=countinversion(arr,start,divide);
15                 count+=countinversion(arr,divide+1,end);
16                 count+=countmerge(arr,start,end,divide);
17             }
18             return count;
19         }
20     
21         private static int countmerge(int[] arr, int start, int end, int divide) {
22             int count = 0;
23             //三个指针的初始化
24             int i = 0;
25             int j = 0;
26             int k = start;
27             boolean flag = false;
28             //创建左右子数组，并赋值
29             int lsize = divide-start+1;
30             int rsize = end-divide;
31             int[] arrL = new int[divide-start+1];
32             int[] arrR = new int[end-divide];
33             for(int n =0;n<lsize;n++){
34                 arrL[n] = arr[n+start];
35             }
36             for(int n =0;n<rsize;n++){
37                 arrR[n] = arr[n+divide+1];
38             }
39             //三个指针移动，进行合并
40             while(k<end+1){
41                 if(arrL[i]>arrR[j]&&flag==false){
42                     count+=lsize-i;
43                     flag = true;
44                 }
45                 if(arrL[i]<arrR[j]){
46                     arr[k]=arrL[i];
47                     k++;
48                     i++;
49                     if(i>=lsize)
50                         break;
51                 }else{
52                     arr[k]=arrR[j];
53                     k++;
54                     j++;
55                     flag = false;
56                     if(j>=rsize)
57                         break;
58                 }
59             }
60             //把剩余的都填进arr
61             while(i<lsize){
62                 arr[k]=arrL[i];
63                 k++;
64                 i++;
65                 //count++;
66             }
67             while(j<rsize){
68                 arr[k]=arrR[j];
69                 k++;
70                 j++;
71             }
72             System.out.print("    本次递归的结果：");
73             Sort.printarr(arr);
74             System.out.println("    本次的计数"+count);
75             return count;
76         }
77     }