P1908 逆序对 树状数组

#include<bits/stdc++.h>
using namespace std;

const int maxn=5e5+10;
#define ll long long
int a[maxn],b[maxn];
ll c[maxn];
int n;
ll ask(int x)
{
    ll ans=0;
    for(; x; x-=x&-x)
        ans+=c[x];
    return ans;
}
void add(int x,int y)
{
    for(; x<=n; x+=x&-x) c[x]+=y;
}
int main()
{
    scanf("%d",&n);
    for(int i=1; i<=n; i++)
    {
        scanf("%d",&a[i]);
        b[i]=a[i];
    }
    sort(b+1,b+1+n);
    int cnt=unique(b+1,b+1+n)-b-1;
    ll ans=0;
    for(int i=1;i<=n;i++)
        a[i]=lower_bound(b+1,b+1+cnt,a[i])-b;
    for(int i=n; i>=1; i--)
    {
        ans=ans+ask(a[i]-1);
        add(a[i],1);
    }
    printf("%lld",ans);
}