Phone number combo - interview

gistfile1.txt

class Solution {
    public List<String> letterCombinations(String digits) {
      
      if (digits == null || digits.length() == 0)
        return new LinkedList<String>();
      
      // local vars
      Map<Integer, String> digitsToChars = new HashMap<Integer, String>();
      digitsToChars.put(0, "");
      digitsToChars.put(1, "");
      digitsToChars.put(2, "abc");
      digitsToChars.put(3, "def");
      digitsToChars.put(4, "ghi");
      digitsToChars.put(5, "jkl");
      digitsToChars.put(6, "mno");
      digitsToChars.put(7, "pqrs");
      digitsToChars.put(8, "tuv");
      digitsToChars.put(9, "wxyz");
      
      List<String> combos = new LinkedList<String>();
      List<Character> prefix = new LinkedList<Character>();
      char[] digitChars = digits.toCharArray();
      
      permutate(combos, prefix, 0, digitChars, digitsToChars);
      
      return combos;
      
    }
  
    private void permutate(final List<String> combos, final List<Character> prefix, int curIndex, 
                           final char[] digitChars, final Map<Integer, String> digitsToChars) {
      
      // Stopping condition
      if (curIndex == digitChars.length) {
        StringBuilder builder = new StringBuilder();
        for (Character charElem : prefix) {
          builder.append(charElem);
        }
        combos.add(builder.toString());
        return;
      }
      
      // Else permutate.
      System.out.println("Current Digit char " + digitChars[curIndex]);
      int curDigit = digitChars[curIndex] - '0';
      System.out.println("Current Digit " + curDigit);
      
      char[] charsForDigit = (digitsToChars.get(curDigit)).toCharArray();
      System.out.println("Chars in Digit " + new String(charsForDigit));
      
      for (char charInDigit : charsForDigit) {
        
        System.out.println("Char in Digit " + charInDigit);
        
        prefix.add(charInDigit);
        
        System.out.println("Chars in Prefix " + prefix);
        
        permutate(combos, prefix, curIndex + 1, digitChars, digitsToChars);
        
        int removeIndex = prefix.size() - 1;
        prefix.remove(removeIndex);
      }
    
    }
}