C++编译错误
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/Scrt1.o: in function _start': (.text+0x24): undefined reference to main'
collect2: error: ld returned 1 exit status

#include <bits/stdc++.h>
using namespace std;
const int mod=1000000007;
long long n,facn=1,ans;
long long a[1000005],p[1000005],pre[1000005];
long long ksm(long long a,long long b)
{
	long long res=1;
	while(b)
	{
		if(b&1)
			res=res*a%mod;
		a=a*a%mod;
		b>>=1;
	}
	return res;
}
long long inv(long long x)
{
	if(x==0)
		return 0;
	return ksm(x,mod-2);
}
int mian(){
	std::ios::sync_with_stdio(false);
	cin.tie(0);cout.tie(0);
	cin>>n;
	for(int i=1;i<=n;i++)
	{
		cin>>a[i];
		facn=facn*i%mod;
	}
	sort(a+1,a+n+1);
	for(int i=1;i<=n;i++)
	{
		if(a[i]==a[i-1])
		{
			p[i]=p[i-1];
			pre[i]=pre[i-1];
		}	
		else
		{
			p[i]=i-1;
			pre[i]=a[i-1];
		}
	}
	for(int i=1;i<=n;i++)
	{
		if(a[i]==a[n])
			break;
		ans=(ans+facn*(inv(n-p[i]))%mod*a[i])%mod;
	}
	cout<<ans;
	return 0;
}