剑指Offer面试题：26.字符串的排列

    public static void Permutation(char[] str)
    {
        if (str == null)
        {
            return;
        }

        Permutation(str, str, 0);
    }

    public static void Permutation(char[] str, char[] begin, int startIndex)
    {
        if (startIndex == str.Length)
        {
            Console.WriteLine(str);
        }
        else
        {
            for (int i = startIndex; i < str.Length; i++)
            {
                char temp = begin[i];
                begin[i] = begin[startIndex];
                begin[startIndex] = temp;

                Permutation(str, begin, startIndex + 1);

                temp = begin[i];
                begin[i] = begin[startIndex];
                begin[startIndex] = temp;
            }
        }
    }

    public static void TestPortal(string str)
    {
        if (string.IsNullOrEmpty(str))
        {
            Console.WriteLine("Test for NULL begins:");
            Permutation(null);
        }
        else
        {
            Console.WriteLine("Test for {0} begins:", str);
            Permutation(str.ToCharArray());
        }

        Console.WriteLine();
    }

    public static void Test1()
    {
        TestPortal(null);
    }

    public static void Test2()
    {
        string str = "";
        TestPortal(str);
    }

    public static void Test3()
    {
        string str = "a";
        TestPortal(str);
    }

    public static void Test4()
    {
        string str = "ab";
        TestPortal(str);
    }

    public static void Test5()
    {
        string str = "abc";
        TestPortal(str);
    }