Generiranje kombinacij znakov

        static void Main(string[] args)
        {
            List<char> ll = new List<char>();
            ll.Add('a');
            char[] allowedCharacters = new char[] { 'a', 'b', 'c' };
            IEnumerable<string> gg = GetStringPermutations(allowedCharacters);
            Console.Out.WriteLine(gg.Skip(6).First());
            Console.In.ReadLine();
        }

        static IEnumerable<string> GetStringPermutations(char[] allowedCharacters)
        {
            List<char> currentString = new List<char>();
            currentString.Add(allowedCharacters[0]);
            yield return new string(currentString.ToArray());
            while (true)
            {
                if (allowedCharacters.Length == 1)
                {
                    currentString.Add(allowedCharacters[0]);
                }
                else
                {
                    currentString = FlipCharacters(currentString, allowedCharacters);
                }
                yield return new string(currentString.ToArray());
            }
        }


        static List<char> FlipCharacters(List<char> currentString, char[] allowedCharacters)
        {
            // nekaj pomoznih spremenljivk
            char lastAllowed = allowedCharacters[allowedCharacters.Length - 1];
            char firstAllowed = allowedCharacters[0];
            // zacnemo s prvim znakom od zadaj
            char firstCharacter = currentString[currentString.Count - 1];
            // ga spremenimo v naslednji znak
            firstCharacter = FindNextCharacter(firstCharacter, allowedCharacters);
            currentString[currentString.Count - 1] = firstCharacter;
            // ce imamo vec znakov in ce je prvi znak od zadaj prisel okoli
            if (currentString.Count > 1 && firstCharacter == firstAllowed)
            {
                // obracamo znake, dokler ne obrnemo enega, ki ni sel okoli
                for (int i = currentString.Count - 2; i >= 0; i--)
                {                    
                    if (currentString[i] == lastAllowed)
                    {
                        currentString[i] = firstAllowed;
                        // ce so vsi sli okoli dodamo nov znak
                        if (i == 0)
                        {
                            currentString.Add(firstAllowed);
                        }
                    }
                    else
                    {
                        currentString[i] = FindNextCharacter(currentString[i], allowedCharacters);
                        break;
                    }
                }
            }
            else if (currentString.Count == 1 && firstCharacter == firstAllowed)
            {
                // samo en znak in ta se je obrnil
                currentString.Add(firstAllowed);
            }
            return currentString;
        }


        static char FindNextCharacter(char c, char[] allowedCharacters)
        {
            if (c == allowedCharacters[allowedCharacters.Length - 1])
            {
                return allowedCharacters[0];
            }
            for (int i = 0;i < allowedCharacters.Length;i++)
            {
                if (c == allowedCharacters[i])
                {
                    return allowedCharacters[i + 1];
                }
            }
            return allowedCharacters[0];
        }