### LeetCode - 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.

`package recursion;import java.util.ArrayList;import java.util.List;public class LetterCombinationsOfAPhoneNumber {    public List<String> letterCombinations(String digits) {        String[] map = { " ", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };        List<String> res = new ArrayList<String>();        if (digits == null || digits.length() == 0) return res;        res.add("");        return DFS(map, res, digits, 0);    }        private List<String> DFS(String[] map, List<String> subRes, String digits, int pos) {        if (pos == digits.length()) return subRes;        int index = digits.charAt(pos) - '0';        List<String> res = new ArrayList<String>();        for (String s : subRes) {            for (int j = 0; j < map[index].length(); ++j) {                res.add(s + map[index].charAt(j));            }        }                return DFS(map, res, digits, ++pos);    }        public static void main(String[] args) {        // TODO Auto-generated method stub        LetterCombinationsOfAPhoneNumber l = new LetterCombinationsOfAPhoneNumber();        System.out.println(l.letterCombinations("23"));    }}`