// Example 5-1：函數 fac()計算階乘n!
int fac(int n)
{
	int result = 1;
	if(n<0)
		return -1;
	else if(n == 0)
		return 1;
	while (n>1)
	{
		result *= n;
		n--;
	}
	return result;
}
// Example 5-2：測試階乘計算函數的主程序
#include <iostream.h>
void main()
{
	int n;
	cout << "Please input a number n to calculte n! ：";
	cin  >> n;
	cout << n << "! = " << fac(n) << endl;
}