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>      /// 排列循環(huán)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>      /// 排列堆棧方法      /// </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>      /// 組合      /// </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);      }    }  }