【LeetCode】 017. 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 {    private static String[] map = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};    public List<String> letterCombinations(String digits) {        List<String> list = new LinkedList<String>();        if (digits.isEmpty()) {            return list;        }        helper(digits, "", list);        return list;    }        private void helper(String digits, String prefix, List<String> list) {        if (prefix.length() == digits.length()) {            list.add(prefix);            return;        }        char[] array = map[digits.charAt(prefix.length()) - '0'].toCharArray();        for (int i = 0; i < array.length; i++) {            helper(digits, prefix + array[i], list);        }    }}