[LC] 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.

A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

Time: O(3^N)

class Solution {
    private String[] letters = {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    public List<String> letterCombinations(String digits) {
        List<String> res = new ArrayList<>();
        if (digits == null || digits.length() == 0) {
            return res;
        }
        helper(res, "", digits, 0);
        return res;
    }
    
    private void helper(List<String> res, String s, String digits, int index) {
        if (s.length() == digits.length()) {
            res.add(s);
            return;
        }
        String curLetter = letters[digits.charAt(index) - '0'];
        // loop through current letters[index] 
        for (int i = 0; i < curLetter.length(); i++) {
            helper(res, s + curLetter.charAt(i), digits, index + 1);
        }
    }
}