递归中的阶乘

#include<stdio.h>
int fac(int n)
{
 int f;
 if(n<0)                                                   //n不能小于0
  printf("n小于0，data error!");
 else
  if(n==0||n==1)                                        //当n=1或0时，n!=1
   f=1;
  else
   f=fac(n-1)*n;                                     //当n大于1时，n!=n*(n-1)!
 return f;
}
int main()
{
 int n,sum;
 printf("please enter integer number:");
 scanf("%d",&n);
 sum=fac(n);
 printf("sum=%d\n",sum);
 return 0;
}