17. Letter Combinations of a Phone Number

LetterCombinationsofPhoneNumber.java

class Solution {
    
    // 17. Letter Combinations of a Phone Number
    // https://leetcode.com/problems/letter-combinations-of-a-phone-number/
    // https://leetcode.com/problems/letter-combinations-of-a-phone-number/discuss/8109/My-recursive-solution-using-Java/222635
    
    private static final String[] KEYS = { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };
    
    public List<String> letterCombinations(String digits) {
    		List<String> ret = new LinkedList<String>();
            if (digits.length() == 0) 
                return ret;
    		combination(new StringBuilder(), digits, 0, ret);
    		return ret;
    }
    
    private void combination(StringBuilder prefix, String digits, int offset, List<String> ret) {
    		if (offset >= digits.length()) {
    			ret.add(prefix.toString());
    			return;
    		}
    		String letters = KEYS[(digits.charAt(offset) - '0')];
    		for (int i = 0; i < letters.length(); i++) {
                prefix.append(letters.charAt(i));
    			combination(prefix, digits, offset + 1, ret);
                prefix.deleteCharAt(prefix.length() - 1);
    		}
    }
    
    /*
    https://leetcode.com/problems/letter-combinations-of-a-phone-number/discuss/8109/My-recursive-solution-using-Java/222635 
    Using String - 
    private void combination(String prefix, String digits, int offset, List<String> ret) {
    		if (offset >= digits.length()) {
    			ret.add(prefix);
    			return;
    		}
    		String letters = KEYS[(digits.charAt(offset) - '0')];
    		for (int i = 0; i < letters.length(); i++) {
    			combination(prefix + letters.charAt(i), digits, offset + 1, ret);
    		}
    }
    */
}