ZJNU 1247 归并排序求逆序对

 1 #include <stdio.h>
 2 #include <algorithm>
 3 #include <string.h>
 4 using namespace std;
 5 
 6 
 7 int a[1000005];
 8 int b[1000005];
 9 __int64 ans;
10 
11 void bing(int left,int mid,int right)
12 {
13     int i, j, k;
14     for(i=left;i<=right;i++)
15     b[i]=a[i];
16     i=left;j=mid+1;k=left;
17     while(i<=mid&&j<=right)
18     {
19         if(b[i]<=b[j]) a[k++]=b[i++];
20         else{
21             a[k++]=b[j++];
22             ans+=mid-i+1;
23         }
24     }
25     while(i<=mid) a[k++]=b[i++];
26     while(j<=right) a[k++]=b[j++];
27 }
28 
29 void msort(int left,int right)
30 {
31     if(left<right)
32     {
33         
34         int mid=(left+right)/2;
35         msort(left,mid);
36         msort(mid+1,right);
37         bing(left,mid,right);
38     }
39 }
40 main()
41 {
42     int n, i;
43     scanf("%d",&n);
44     for(i=1;i<=n;i++) scanf("%d",&a[i]);
45     ans=0;
46     msort(1,n);
47     printf("%I64d\n",ans);
48 }