n！

using System;
using System.Collections.Generic;
using System.Text;

namespace ExFactorial
{
    class Factorial
    {
        public double factorial(int num)
        {
            switch (num)
            {
                case 1:
                    return 1;
                default :
                    return num * factorial(num - 1);                    
            }

        }
        static void Main(string[] args)
        {
            Console.WriteLine("输入一个整数：");
            int num=Convert.ToInt32 (Console.ReadLine());
            Factorial f = new Factorial();
            Console.WriteLine("{0}!的值为{1}", num, f.factorial(num));
            Console.ReadLine();
        }
    }
}