第一，二章

#include<stdio.h>
int main(void)
{
 int n;   
  int factorial(int n);

  scanf("%d",&n);
  printf("%d
",factorial(n));

  return 0;
}

int factorial(int n)
{
 int i,fact=1;

 for(i=1;i<=n;i++)
  fact=fact*i;

 return fact;

}

/*"Hello World! */
# include<stdio.h>

int main(void)

{
 printf("Hello World!
");


 return 0;

}