打印输入字符串的字母所有顺序的排列

        static void PrintSequenceString(string inputStr)
        {
            if (string.IsNullOrEmpty(inputStr))
                throw new ArgumentNullException(string.Format("{0} is illegal", inputStr));
            
            char[] chr = inputStr.ToCharArray();
            List<string> results = new List<string>();
            Generate(chr, 0, results);
            
            foreach (string s in results)
            {
                Console.WriteLine(s);
            }
        }

        static void Generate(char[] chr, int index, List<string> results)
        {
            if (index == chr.Length)
                results.Add(new string(chr));
            for (int i = index; i < chr.Length; i++)
            {
                swap(chr, i, index);
                Generate(chr, index+1, results);
                swap(chr, index, i);
            }
        }

        static void swap(char[] chr, int i, int j)
        {
            char tmp = chr[i];
            chr[i] = chr[j];
            chr[j] = tmp;
        }