leetcode : Letter Combinations of 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.

思路： 深度优先， 回溯

public class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> result = new ArrayList<String>();
        if(digits == null || digits.length() == 0) {
            return result;
        }
       
        Map<Character,char[]> map = new HashMap<Character, char[]>();
        map.put('1', new char[]{});
        map.put('2', new char[]{'a', 'b', 'c'});
        map.put('3', new char[]{'d', 'e', 'f'});
        map.put('4', new char[]{'g', 'h' , 'i'});
        map.put('5', new char[] { 'j', 'k', 'l' });
        map.put('6', new char[] { 'm', 'n', 'o' });
        map.put('7', new char[] { 'p', 'q', 'r', 's' });
        map.put('8', new char[] { 't', 'u', 'v'});
        map.put('9', new char[] { 'w', 'x', 'y', 'z' });
         StringBuilder sb = new StringBuilder();
         helper(result, digits, map, sb);
         return result;
    }
    
    public void helper(List<String> result, String digits, Map<Character, char[]> map, StringBuilder sb) {
        if(sb.length() == digits.length()) {
            result.add(sb.toString());
            return;
        }
        
        for(char c : map.get(digits.charAt(sb.length()))) {
            sb.append(c);
            helper(result, digits, map, sb);
            sb.deleteCharAt(sb.length() - 1);
        }
        
    }
}