Letter Combinations of a Phone Number

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Simple backtracking.

public List<String> letterCombinations(String digits) {
        List<String> results = new ArrayList<String>();
        if (digits==null||digits.isEmpty()) {
            return results;
        }

        String[] letters = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};        

        combination(letters, results, new StringBuilder(), 0, digits);
        return results;
    }

    public void combination(String[] letters, List<String> results, StringBuilder sb, int level, String digits) {
        if (level==digits.length()) {
            results.add(sb.toString());
            return;
        }

        int currentDigit = digits.charAt(level)-'0';
        for (int i=0; i<letters[currentDigit].length(); i++) {
            sb.append(letters[currentDigit].charAt(i));
            combination(letters, results, sb, level+1, digits);
            sb.deleteCharAt(sb.length()-1);
        }    
    }