裸題，不多解釋。

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<algorithm>
 4 #include<cstring>
 5 using namespace std;
 6 #define maxn 500005 
 7 int a[maxn],b[maxn],c[maxn];
 8 int n;
 9 long long ans;
10 int lowbit(int x){
11     return x&(-x);
12 }
13 void updata(int
 
 x){
14     for(int i=x;i<=n;i+=lowbit(i))
15         c[i]+=1;
16 }
17 int ask(int x){
18     int tot=0;
19     for(int i=x;i>=1;i-=lowbit(i))
20         tot+=c[i];
21     return tot;
22 }
23 int main(){
24     while(scanf("%d",&n),n){
25     int cnt=0;
26     for(int i=1;i<=n;i++){
 
27         scanf("%d",&a[i]);
28         b[i]=a[i];
29     }
30     ans=0;
31     sort(a+1,a+n+1);
32     cnt=unique(a+1,a+n+1)-a-1;
33     memset(c,0,sizeof c);
34     for(int i=1;i<=n;i++){
35         b[i]=lower_bound(a+1,a+cnt+1,b[i])-a;
36         updata(b[i]);
37         ans+=i-ask(b[i]);
 
38     }
39     printf("%I64d\n",ans);
40     }
41 } 

POJ2299 樹狀數組求逆序對