树状数组求逆序对

#include<bits/stdc++.h>
using namespace std;
#define ll long long
const int maxn=5e5+10;
int ne[maxn],od[maxn],tmp[maxn],bit[maxn],n,cnt;
inline int lowbit(int x){return x&-x;}
inline int read()

{

    int x=0,f=1; char ch=getchar();

    while(ch<'0'||ch>'9'){if(ch=='-')f=-1; ch=getchar();}

    while(ch>='0'&&ch<='9'){x=x*10+ch-'0'; ch=getchar();}

    return x*f;

}
inline void add(int pos)
{
    while(pos<=cnt)
    {
        ++bit[pos];
        pos+=lowbit(pos);
    }
}
inline ll query(int pos)
{
    ll ans=0;
    while(pos>0)
    {
        ans+=bit[pos];
        pos-=lowbit(pos);
    }
    return ans;
}
int main()
{
    n=read();
    for(int i=1;i<=n;i++)
    {
        od[i]=read();
        tmp[i]=od[i];
    }
    sort(tmp+1,tmp+n+1);
    cnt=unique(tmp+1,tmp+n+1)-tmp-1;
    for(int i=1;i<=n;i++)
        ne[i]=lower_bound(tmp+1,tmp+cnt+1,od[i])-tmp;
    ll ans=0;
   // for(int i=1;i<=n;i++)
    //{
        //cout<<ne[i]<<"  ";
   // }
    //cout<<endl;
    for(int i=1;i<=n;i++)
    {
        add(ne[i]);
        ans+=(query(cnt)-query(ne[i]));
        //cout<<i<<"  "<<query(ne[i]-1)<<endl;
    }
    cout<<ans<<endl;
    return 0;
}