Codevs 1688 求逆序对

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 #include<algorithm>
 5 using namespace std;
 6 int n,m,a[100010]={0},b[100010];
 7 long long ans=0;
 8 void ni(int l,int r)
 9 {
10     if(l==r) return;
11     int mid=(l+r)/2;
12     ni(l,mid);ni(mid+1,r);
13     int i=l,j=mid+1,k=l;
14     while(i<=mid&&j<=r)
15     {
16         if(a[i]>a[j])
17         {
18             b[k++]=a[j++];
19             ans+=mid-i+1;
20         }
21         else{
22             b[k++]=a[i++];
23         }
24     }
25      while(i<=mid) b[k++]=a[i++];
26      while(j<=r) b[k++]=a[j++];
27      for(int w=l;w<=r;w++) a[w]=b[w];
28 }
29 int main()
30 {
31     scanf("%d",&n);
32     for(int i=1;i<=n;i++)
33     { scanf("%d",&a[i]); }
34     ni(1,n);
35     printf("%lld",ans);
36     return 0;
37 }