Some string manipulations for future use.

1. Using an array of characters, return all possible permutations of this array (without repetitions).

public static List<string> StringPermutations(char[] list)
{
 List<string> result = new List<string>();
 int x=list.Length-1;
 go(list,0,x, result);
 return result;
}

private static void go (char[] list, int k, int m, List<string> result)
{
 int i;
 if (k == m)
 {
  result.Add(new string(list));
 }
 else
 for (i = k; i <= m; i++)
 {
  swap (ref list[k],ref list[i]);
  go (list, k+1, m, result);
  swap (ref list[k],ref list[i]);
 }
}

private static void swap(ref char a, ref char b)
{
 if (a == b) return;
 a ^= b;
 b ^= a;
 a ^= b;
}

Sample usage

List<string> permutations = Helper.StringPermutations(new char[] {'D', 'N', 'A'});

Sample output

DNA
DAN
NDA
NAD
AND
ADN

2. Using an array of characters ("alphabet"), return all possible words generated from this alphabet of a specified length

public static IEnumerable<String> GetWordsWithRepetition(Int32 length, char[] alphabet)
{
 if (length <= 0)
  yield break;

 for(int i = 0; i < alphabet.Length; i++) // (Char c = 'A'; c <= 'Z'; c++)
 {
  char c = alphabet[i];
  if (length > 1)
  {
   foreach (String restWord in GetWordsWithRepetition(length - 1, alphabet))
    yield return c + restWord;
  }
  else
   yield return "" + c;
 }
}

3. Further can be used to get full "dictionary" with all possible words up to a specified length

public static string ALPHABET = "D N A";

public static List<string> Dictionary(int length)
{
 char[] alphabet = Helper.AlphabetFromString(ALPHABET);

 List<string> final = new List<string>();

 for (int i = 1; i <= length; i++)
 {
  List<string> result = Helper.GetWordsWithRepetition(i, alphabet).ToList();
  final.AddRange(result);
 }
 return final;
}

public static char[] AlphabetFromString(string input)
{
 string[] split = input.Split(' ');
 char[] alphabet = new char[split.Count()];
 for (int i = 0; i < alphabet.Length; i++)
 {
  alphabet[i] = split[i][0];
 }
 return alphabet;
}

4. Further can be used to sort the words of the dictionary according to the alphabet provided using a comparer

public static int WordComparer(string one, string two)
{
 char[] alphabet = AlphabetFromString(ALPHABET);

 int len = Math.Min(one.Length, two.Length);
 for (int i = 0; i < len; i++)
 { 
  int posOne = Array.IndexOf(alphabet, one[i]);
  int posTwo = Array.IndexOf(alphabet, two[i]);
  if (posOne == posTwo)
  {
   continue;
  }
  else if(posTwo > posOne)
  {
   return -1;
  }
  return 1;
 }
 return two.Length > one.Length ? -1 : 1;
}

Sample usage

List<string> final = Dictionary(3).Sort(WordComparer);

Sample output

D
DD
DDD
DDN
DDA
DN
DND
DNN
DNA
DA
DAD
DAN
DAA
N
ND
NDD
NDN
NDA
NN
NND
NNN
NNA
NA
NAD
NAN
NAA
A
AD
ADD
ADN
ADA
AN
AND
ANN
ANA
AA
AAD
AAN
AAA

by Evgeny. Also posted on my website