using System;
using System.Collections.Generic;
 
class App
{
  static public void Main(string[] args)
  {
    const int N = 6;
 
    // 素朴にループで
    Console.WriteLine(Fact1(N));
 
    // 素朴に再帰で
    Console.WriteLine(Fact2(N));
 
    // コールスタックを自前で用意して再帰っぽく
    Console.WriteLine(Fact3(N));
  }
 
  // 素朴にループ
  static int Fact1(int n)
  {
    int prod = 1;
    for (int i = 0; i < n; i++)
    {
      prod *= (i + 1);
    }
    return prod;
  }
 
  // 素朴に再帰
  static int Fact2(int n)
  {
    if (n == 0) return 1;
    return n * Fact2(n - 1);
  }
 
  // コールスタックに積まれる情報
  class Frame
  {
    public int Arg { get; set; }
    public int Ret { get; set; }
    public Frame(int arg) { Arg = arg; }
  }
 
  // C#が提供するコールスタックを使わず
  // スタックを自前で用意
  static int Fact3(int n)
  {
    // なんちゃってコールスタック
    Stack<Frame> frames = new Stack<Frame>();
 
    Frame f = new Frame(n);
    while(true)
    {
      if (f.Arg == 0)
      {
        // 再帰の出口
        f.Ret = 1;
        break;
      }
      else
      {
        // 再帰呼び出し
        frames.Push(f);
        f = new Frame(f.Arg - 1);
      }
    }
 
    // 残りの掛け算
    while (0 < frames.Count)
    {
      Frame peek = frames.Peek();
      peek.Ret = f.Ret * peek.Arg;
      f = frames.Pop();
    }
    return f.Ret;
  }
}
 

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