樹狀數組求逆序數+離散化

http://poj.org/problem?id=2299

#include<algorithm>
#include<cstdio>
#include<cstring>
using namespace std;
typedef long long ll;
const int maxn=1000010;
int n,c[maxn];
int lowbit(int x)
{
    return x&(-x);
}
ll query(int x)
{
    ll res=0;
    while(x)
    {
        res+=c[x];
        x-=lowbit(x);
    }
    return res;
}
void add(int x,int val)
{
    while(x<=n)
    {
        c[x]+=val;
        x+=lowbit(x);
    }
}
struct node
{
    int val,id;
};
bool cmp(node a,node b)
{
    return a.val<b.val;
}
node a[maxn];
int b[maxn];
int main()
{
    //int x,y;
    while(scanf("%d",&n))
    {
        if(n==0)
            break;
        for(int i=1;i<=n;i++)
        {
            scanf("%d",&a[i].val);
            a[i].id=i;
        }
        sort(a+1,a+1+n,cmp);
        int f=1;
        b[a[1].id]=1;
        for(int i=2;i<=n;i++)
        {
            if(a[i].val==a[i-1].val)
                {
                    b[a[i].id]=b[a[i-1].id];
                }
            else
            {
                b[a[i].id]=++f;
            }
        }
        ll ans=0;
        memset(c,0,sizeof(c));
        for(int i=n;i>=1;i--)
        {
            add(b[i],1);
            ans+=query(b[i]-1);
        }
        printf("%lld\n",ans);
    }
}