归并排序

 1 import java.util.*;
 2 import static java.lang.System.*;
 3 
 4 public class Main{
 5     static Scanner in = new Scanner(System.in);
 6     
 7     static int sum;
 8     static void merge(int a[],int first,int mid,int last,int temp[])
 9     {
10         int i=first,j=mid+1;
11         int n=mid,m=last;
12         int k=0;
13         while(i<=n&&j<=m)
14         {
15             if(a[i]>a[j])
16             {
17                 temp[k++]=a[j++];
18                 sum+=(j-first-k);//j-first表示该组中a[j]前面有几个数,k表示归并后a[j]在该组中的位置
19             }
20             else
21                 temp[k++]=a[i++];
22         }
23         
24         while(i<=n)    temp[k++]=a[i++];
25         while(j<=m) temp[k++]=a[j++];
26         for(int l=0;l<k;l++)
27         {
28             a[first+l]=temp[l];
29         }    
30     }
31     static void MergeSort(int a[],int first,int last,int temp[])
32     {
33         if(first<last)
34         {
35             int mid=(first+last)/2;
36             MergeSort(a,first,mid,temp);
37             MergeSort(a,mid+1,last,temp);
38             merge(a,first,mid,last,temp);
39         }
40     }
41     
42     public static void main(String[] args)
43     {
44         int a[] = {3,2,9,7,5,4};
45         int temp[]=new int[20];
46         MergeSort(a,0,a.length-1,temp);
47         for(int elem:a)
48             out.print(elem);
49         out.println("\n序列的逆序数为:"+sum);
50     }
51 }