打印1至n个自然数的全排列的一种实现（递归，有点tricky）

　　缘起老赵的在CSDN的学生大本营里的一个题：《您有没有尝试过打印出一个集合的全排列》。大半夜的看到，就关了电脑，躺着床上想着怎么解。突然蹦出个点子，用一组小集合验证了一下可行性后，实在按奈不住那种似发现新大陆般的激动，就爬起来，敲了下去。真正的代码倒不多，自己也惊讶竟然找到个如此tricky的方法。如果您以前没有做过这个题，也欢迎您去老赵的那个页面看一看、想一想您的解法。我的这部分拙作，就先折叠起来吧～

　　图省事，代码就没有加注释了。运算结果在n小于等于3的情况都验证了，再往后实在是精力有限……

using System;
using System.Collections.Generic;

namespace MathProblems.Permutation
{
    class Program
    {
        static void Main(string[] args)
        {
            bool parsingSuccess = false;
            do
            {
                Console.WriteLine("Please input the upper bound of numbers to be permutated (lower bound was set to 1): ");
                string upperBoundString = Console.ReadLine();
                int upperBound;
                parsingSuccess = Int32.TryParse(upperBoundString, out upperBound);
                if (parsingSuccess)
                    PrintPermutation(upperBound);
            }
            while (parsingSuccess);
        }

        private static Queue<int> emptySlots;
        private static int[] numberSlots;

        static void PrintPermutation(int upperBound)
        {
            Console.WriteLine("{0} : Starting permutation of numbers between 1 and {1}:", Environment.TickCount, upperBound);

            emptySlots = new Queue<int>(upperBound);
            for (int i = 0; i < upperBound; i++)
            {
                emptySlots.Enqueue(i);
            }

            numberSlots = new int[upperBound];

            PickSlot(upperBound);

            Console.WriteLine("{0} : Ending permutation.", Environment.TickCount);
        }

        static void PickSlot(int number)
        {
            if (number == 0)
            {
                PrintArray(numberSlots);
            }
            else
            {
                int emptySlotCount = emptySlots.Count;
                for (int i = 0; i < emptySlotCount; i++)
                {
                    int slotPicked = emptySlots.Dequeue();
                    numberSlots[slotPicked] = number;

                    PickSlot(number - 1);

                    emptySlots.Enqueue(slotPicked);
                }
            }
        }

        static void PrintArray(int[] array)
        {
            return;
            for (int i = 0; i < array.Length; i++)
            {
                Console.Write(array[i]);
                Console.Write(" ");
            }
            Console.WriteLine();
        }
    }
}