leetcode 17: 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.

Input:Digit string "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.

public class Solution {
    public List<String> letterCombinations(String digits) {
        String[] map = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        
        List<String> res = new ArrayList<String>();
        
        rec(res, new StringBuilder(), digits, map, 0);
        return res;
    }
    
    private void rec(List<String> res, StringBuilder sb, String digits, String[] map, int level){
        if(level==digits.length() ){
            res.add(sb.toString());
            return;
        }
        String s = map[digits.charAt(level)-'2'];
        for(int i=0; i<s.length(); i++) {
            sb.append(s.charAt(i));
            rec(res, sb, digits, map, level+1);
            sb.deleteCharAt(sb.length()-1);
        }
    }
}