C# 排列与子排列

        static StringCollection Permutations(string input)
        {
            if (input == null)
            {
                throw new Exception("input can't be null");
            }
            StringCollection results = new StringCollection();
            char firstChar = ' ';
            string remainString = "";
            string temp = "";
            if (input.Length<=1)
            {
                results.Add(input);
                return results;
            }

            for (int i = 0; i < input.Length; i++)
            {
                firstChar = input[i];
                remainString = input.Remove(i, 1);
                StringCollection remainPermutationsString = Permutations(remainString);
                for (int j = 0; j < remainPermutationsString.Count; j++)
                {
                    temp = firstChar + remainPermutationsString[j];
                    results.Add(temp);
                }
            }
            return results;
        }

        static StringCollection Anagram(string input)
        {
            StringCollection results = Permutations(input);
            results.Remove(input);
            return results;
        }

        static bool IsAnagram(string str1, string str2)
        {
            if (str1== null || str2 == null)
            {
                throw new Exception("input string can't be null");
            }
            StringCollection anagram = Anagram(str1);
            foreach (string item in anagram)
            {
                if (item.Equals(str2))
                {
                    return true;
                }
            }
            return false;
        }