Letter Combinations of a Phone Number

LeetCode Q 17 - Letter Combinations of a Phone Number

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent.

Example:
Input: "23" ;Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note: Although the above answer is in lexicographical order, your answer could be in any order you want.

Solution : Backtracking

Code:

private static final String[] KEYS = { "", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };
private List<String> res;
public List<String> letterCombinations(String digits) {
	res = new ArrayList<>();
	if (digits == null || digits.length() == 0) return res;
	backtrack(digits, 0, "");
	return res;
}

private void backtrack(String digits, int pos, String curr) {
	// corner cases
	if (pos == digits.length()) {
		res.add(curr); return;
	}
	
	String letters = KEYS[digits.charAt(pos) - '0'];
	for (char letter: letters.toCharArray())
		backtrack(digits, pos + 1, curr + letter);
}