刚退役OIer出题遇到困难

#include<bits/stdc++.h>
#define ll long long
#define mod 10000007
using namespace std;
ll f[1000005];
int main(){
	freopen("3.in","r",stdin);
	//freopen("3.out","w",stdout);
	int n;cin>>n;
	cout<<n<<endl;
	for(int i=1;i<=n;i++){
		ll x;cin>>x;
		if(i==1) f[i]=x;
		else f[i]=((f[i-1]*x)%mod)+f[i-1]+x;
		f[i]%=mod;
		if(i%10000==0) cout<<i<<" "<<f[i]<<" "<<x<<endl;
	}
	cout<<f[n]<<endl;
}

#include<bits/stdc++.h>
using namespace std;
int main(){
	int n=1000000;
	srand(time(0));
	freopen("3.in","w",stdout);
	cout<<n<<endl;
	for(int i=1;i<=n;i++){
		cout<<rand()<<' ';
	}
}