#codingexercise 

Generate Lexicographically ordered combinations of a string  

        public static void Combine(ref String numbers, ref StringBuilder candidate, ref SortedList<String, String> combinations, int level, int start) 

        { 

            for (int i = start; i < numbers.Length; i++) 

            { 

                if (candidate.ToString().Contains(numbers[i]) == false) 

                { 

                    candidate[level] = numbers[i]; 

                    combinations.Add(candidate.ToString(), candidate.ToString()); 

                    if (i < numbers.Length - 1) 

                        Combine(ref numbers, ref candidate, ref combinations, level + 1, start + 1); 

                    candidate[level] = '\0'; 

                } 

            } 

        } 

        static void ToPrint(SortedList<String, String> combinations, string prefix) 

        { 

            Console.WriteLine("---------------------------------------"); 

            for (int i = 0; i < combinations.Count; i++) 

                Console.WriteLine(prefix + combinations.ElementAt(i).Key); 

            Console.WriteLine("---------------------------------------"); 

        } 

        static void Main(string[] args) 

        { 

            var a = "321"; 

            var b = new StringBuilder(); 

            for (int i = 0; i < a.Length; i++) 

                b.Append('\0'); 

            int start = 0; 

            int level = 0; 

            var combinations = new SortedList<String, String>(); 

            Combine(ref a, ref b, ref combinations, level, start); 

            var prefix = String.Empty; 

            ToPrint(combinations, prefix); 

        } 

--------------------------------------- 

1 

2 

21 

3 

31 

32 

321 

--------------------------------------- 

If the starting input string is already sorted from left to right, then the following permutations do not require a SortedList and the additions to a dictionary are already sorted and the results are the same for each prefix

        static void Main(string[] args)
        {
            var a = "123";
            var b = new StringBuilder();
            var used = new bool[a.Length];
            int start = 0;
            int end = a.Length - 1;
            var permutations = new Dictionary<String, String>();
            for ( int i = end ; i>= start; i--){
                var prefix = a.Substring(0, start);
                Permute(a, b, used, start, end, ref permutations);
                ToPrint(permutations, prefix);
                permutations = new Dictionary<String, String>();
           }
        }