改错:计算阶乘，found下的为错误

Question

计算阶乘
#include <stdio.h>
int main()
{
/FOUND/
int fac(n);
int n;
int y;
printf("input an integer number:");
scanf("%d",&n);
/FOUND/
printf("%lf\n",n);
y=fac(n);
printf("%d!=%d\n",n,y);
return 0;
}

int fac(int n)
{
int f;
if(n<0)
printf("n<0,data error!");
/FOUND/
if(n==0||n==1)
f=1;
else
f=fac(n-1)*n;
return(f);

Answer 1

#include <stdio.h>
int main()
{
int fac(int n);
int n;
int y;
printf("input an integer number:");
scanf("%d",&n);
printf("%d\n",n);
y=fac(n);
printf("%d!=%d\n",n,y);
return 0;
}

int fac(int n)
{
int f;
if(n<0)
printf("n<0,data error!");
if(n==0 || n==1)
f=1;
else
f=fac(n-1)*n;
return f ;
}