POJ2299 树状数组求逆序对

裸题，不多解释。

 #include<iostream>
 #include<cstdio>
 #include<algorithm>
 #include<cstring>
 using namespace std;
 #define maxn 500005
 int a[maxn],b[maxn],c[maxn];
 int n;
 long long ans;
 int lowbit(int x){
     return x&(-x);
 }
 void updata(int x){
     for(int i=x;i<=n;i+=lowbit(i))
         c[i]+=;
 }
 int ask(int x){
     int tot=;
     for(int i=x;i>=;i-=lowbit(i))
         tot+=c[i];
     return tot;
 }
 int main(){
     while(scanf("%d",&n),n){
     int cnt=;
     for(int i=;i<=n;i++){
         scanf("%d",&a[i]);
         b[i]=a[i];
     }
     ans=;
     sort(a+,a+n+);
     cnt=unique(a+,a+n+)-a-;
     memset(c,,sizeof c);
     for(int i=;i<=n;i++){
         b[i]=lower_bound(a+,a+cnt+,b[i])-a;
         updata(b[i]);
         ans+=i-ask(b[i]);
     }
     printf("%I64d\n",ans);
     }
 }