class Program
{
    static void Main(string[] args)
    {
      Console.WriteLine("请输入一个数");
      int number = Convert.ToInt32(Console.ReadLine());
      double result = JieCheng(number);
      Console.WriteLine(number.ToString() + "的阶乘结果是：" + result.ToString());
      Console.ReadKey();
    }
 
    public static double JieCheng(int number)
    {
      if (number == 0)
      {
        return 0;
      }
 
      //初始值必须设置为1
      double result = 1;
 
      for (int i = number; i >= 1; i--)
      {
        result = result*i;
      }
      return result;
    }
}

public static double JieCheng(int number)
{
  if (number == 0)
  {
 return 1;
  }

  return number * JieCheng(number - 1);
}