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 letterCombinations(String digits) {
List 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);  
    }  
}  

}