这是题解的O（n^2）的做法，为什么会出现只有25

#include<bits/stdc++.h>
#define itn int
#define tin int
#define nit int
#define tni int
#define nti int
#define scnaf scanf
#define ptrinf printf
#define icn cin
#define cni cin
#define inc cin
#define nci cin
#define nic cin
#define cuot cout
#define ocut cout
#define fro for
using namespace std;
long long n,a[1000001],ans=1,f[1000001];
const long long MOD=1e9+7;
int main()
{
// 	freopen(".in","r",stdin);
 //	freopen(".out","w",stdout);
 	cin>>n;
 	f[0]=1;
 	for(int i=1;i<=n;i++)
 	{
 		f[i]=f[i-1]*(long long)(pow(2,i)-1)%MOD;
	}
	cout<<f[n]%MOD;
 	return 0;
}