用C语言编写n!

#include "iostream.h"
long fax(int n);
int main()
{
int n;
long m;
cout<<"请输出一个数："<<endl;
cin>>n;
m=fax(n);
cout<<n<<"!="<<m<<endl;
return 0;
}
long fax(int n)
{
long f;
if(n<0)
{
cout<<"n<0其值为负数"<<endl;
f=-1;
}
else if(n==0|n==1)
f=1;
else f=fax(n-1)*n;
return f;
}

what?

sum=1
for(i=2;i<=n;i++)
sum*=i