树状数组求逆序数

poj 2299 树状数组求逆序数
题目链接：http://poj.org/problem?id=2299

#include <stdio.h>
#include <string.h>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <string>
#include <math.h>
#include <stdlib.h>
using namespace std;
#define LL long long
struct node
{
    LL v,id;
}p[500005];
LL f[500005],n;
LL d[500005];
bool cmp(node a,node b)
{
    return a.v<b.v;
}
int lowbit(int x)
{
    return x&(-x);
}
void update(int x)
{
    while(x<=n)
    {
        f[x]++;
        x+=lowbit(x);
    }
    return;
}
LL sum(int x)
{
    LL s=0;
    while(x)
    {
        s+=f[x];
        x-=lowbit(x);
    }
    return s;
}
int main()
{
    LL i,j,k,m;
    while(scanf("%lld",&n)!=EOF)
    {
        if(n==0) break;
        memset(f,0,sizeof(f));
        memset(d,0,sizeof(d));
        for(i=1;i<=n;i++)
        {
            scanf("%lld",&p[i].v);
            p[i].id=i;
        }
        sort(p+1,p+n+1,cmp);
        for(i=1;i<=n;i++)
            d[p[i].id]=i;
        LL ans=0;
        for(i=1;i<=n;i++)
        {
            update(d[i]);
            ans+=(i-sum(d[i]));
        }
        printf("%lld
",ans);
    }
    return 0;
}