大数据求逆序数（树状数组）

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
typedef long long ll;
using namespace std;
ll sum[500050],n;
struct node{
    ll lf;
    ll rg;
}a[500050];
ll cmp(node x,node y)
{
    if(x.lf==y.lf)
        return x.rg<y.rg;
    return x.lf<y.lf;
}
ll lowbit(ll x)
{
    return x&(-x);
}
ll Point_change(ll x)
{
    while(x<=n)
    {
        sum[x]=sum[x]+1;
        x=x+lowbit(x);
    }
}
ll get_sum(ll x)
{
    ll res=0;
    while(x>0)
    {
        res=res+sum[x];
        x=x-lowbit(x);
    }
    return res;
}
int main()
{
    ll sm;
    while(scanf("%lld",&n))
    {
        if(n==0)
            break;
            memset(sum,0,sizeof(sum));
        for(ll i=1;i<=n;i++)
        {
            scanf("%lld",&a[i].lf);
            a[i].rg=i;
        }
        sort(a+1,a+n+1,cmp);
        sm=0;
        for(ll i=1;i<=n;i++)
        {
            ll st=get_sum(a[i].rg);
            ll ed=get_sum(n);
            Point_change(a[i].rg);
            sm=sm+ed-st;
        }
        printf("%lld\n",sm);
    }
}