树状数组求逆序对。

#include<bits/stdc++.h>
using namespace std;
const int mod=99999997;
int n,d[100001],e[100001],f[100001];
long long a[100001],b[100001],c[100001];
bool cmpe(int x,int y)
{
    return a[x]<a[y];
}
bool cmpf(int x,int y)
{
    return b[x]<b[y];
}
long long query(int x)
{
	long long ans=0;
	for (int i=x;i>0;i-=i&-i)
		ans=(ans+c[i])%mod;
	return ans;
}
void add(int x,long long k)
{
	for (int i=x;i<=n;i+=i&-i)
		c[i]=(c[i]+k)%mod;
	return;
}
int main()
{
    scanf("%d",&n);
    for (int i=1;i<=n;++i)
        scanf("%lld",&a[i]),e[i]=i;
    for (int i=1;i<=n;++i)
        scanf("%lld",&b[i]),f[i]=i;
    sort(e+1,e+n+1,cmpe);
    sort(f+1,f+n+1,cmpf);
    for (int i=1;i<=n;++i)
        d[e[i]]=f[i];
    long long ans=0;
    for (int i=n;i>0;--i)
    {
        add(d[i],1);
        ans=(ans+query(d[i]-1))%mod;
    }
    printf("%lld",ans);
    return 0;
}