using System;  
using System.Collections.Generic;  
namespace Test  
{  
  class Program  
  {  
    static void Main(string[] args)  
    {  
      Console.WriteLine(P1(6, 3));  
      Console.WriteLine(P2(6, 3));  
      Console.WriteLine(C(6, 2));  
    }  
 
    /// <summary>  
    /// 排列循环方法  
    /// </summary>  
    /// <param name="N"></param>  
    /// <param name="R"></param>  
    /// <returns></returns>  
    static long P1(int N, int R)  
    {  
      if (R > N || R <= 0 || N <= 0 ) throw new ArgumentException("params invalid!");  
      long t = 1;  
      int i = N;  
        
      while (i!=N-R)  
      {  
        try 
        {  
          checked 
          {  
            t *= i;  
          }  
        }  
        catch 
        {  
          throw new OverflowException("overflow happens!");  
        }  
        --i;  
      }  
      return t;  
    }  
 
    /// <summary>  
    /// 排列堆栈方法  
    /// </summary>  
    /// <param name="N"></param>  
    /// <param name="R"></param>  
    /// <returns></returns>  
    static long P2(int N, int R)  
    {  
      if (R > N || R <= 0 || N <= 0 ) throw new ArgumentException("arguments invalid!");  
      Stack<int> s = new Stack<int>();  
      long iRlt = 1;  
      int t;  
      s.Push(N);  
      while ((t = s.Peek()) != N - R)  
      {  
        try 
        {  
          checked 
          {  
            iRlt *= t;  
          }  
        }  
        catch 
        {  
          throw new OverflowException("overflow happens!");  
        }  
        s.Pop();  
        s.Push(t - 1);  
      }  
      return iRlt;  
    }  
 
    /// <summary>  
    /// 组合  
    /// </summary>  
    /// <param name="N"></param>  
    /// <param name="R"></param>  
    /// <returns></returns>  
    static long C(int N, int R)  
    {  
      return P1(N, R) / P1(R, R);  
    }  
  }  
}