CODE

public class Solution {
  public List<string> LetterCombinations(string digits) {
    buildStrings(digits, 0, "");
    return combos;
  }

  List<string> combos = new List<string>();
  void buildStrings(string digits, int index, string current)
  {
    if (index > digits.Length-1) {
      if (current != "")
        combos.Add(current); 
      return;
    }

    string letters = returnPattern(digits[index]);

    foreach (var l in letters)
    {
      current += l;
      buildStrings(digits, index+1, current);           
      current = current.Substring(0, current.Length-1);
    }
  }

  string returnPattern(char d)
  {
    string res = "";
    switch(d)
    {
      case '2': 
        res = "abc";
        break;
      case '3':
        res = "def";
        break;
      case '4': 
        res = "ghi";
        break;
      case '5':
        res = "jkl";
        break;
      case '6': 
        res = "mno";
        break;
      case '7':
        res = "pqrs";
        break;
      case '8': 
        res = "tuv";
        break;
      case '9':
        res = "wxyz";
        break;
      default:
        res = "";
        break;
    }
    return res;
  }
}