Factorial

Factorial  计算阶乘

In mathematics, the factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n. For example: 5! = 5 * 4 * 3 * 2 * 1 = 120. By convention the value of 0! is 1.

Write a function to calculate factorial for a given input. If input is below 0 or above 12 throw an exception of type ArgumentOutOfRangeException (C#) or IllegalArgumentException (Java).

More details about factorial can be found here: http://en.wikipedia.org/wiki/Factorial

刚开始没有注意题目的提示，可以直接判断大于12，就溢出了。想到了使用checked关键字来检查是否溢出。

https://msdn.microsoft.com/zh-cn/library/74b4xzyw.aspx   checked关键字的用法

public static class Kata
    {
        public static int Factorial(int n)
        {
            try
            {
                return Recursion(n);
            }
            catch
            {
                throw;
            }
        }

        public static int Recursion(int n)
        {
            if (n < 0)
            {
                throw new ArgumentOutOfRangeException();
            }
            try
            {

                int factorial = 1;
                if (n >= 2)
                {
                    checked
                    {
                        factorial = n * Recursion(n - 1);
                    }
                }
                return factorial;
            }
            catch
            {
                throw new ArgumentOutOfRangeException();
            }
        }
    }

其他人的解法：

public static int Factorial(int n)
  {
    if(n < 0 || n > 12)
      throw new ArgumentOutOfRangeException();
    return n > 0 ? n * Factorial(n - 1) : 1;
  }

using System;
using System.Linq;

public static class Kata
{
  public static int Factorial(int n)
  {
    if(n < 0 || n > 12) throw new ArgumentOutOfRangeException();

    return Enumerable.Range(1, n).Aggregate(1, (x,y) => x * y);
  }
}