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.

Use a hashmap to store the digits and corresponding strings. Similar to subsets, permutations, use recursion with iteration to find all possible results.

public class Solution {    public ArrayList<String> letterCombinations(String digits) {        ArrayList<String> res = new ArrayList<String>();        if (digits == null) {            return res;        }                HashMap<Character, String> map = new HashMap<Character, String>();        map.put('1', "");        map.put('2', "abc");        map.put('3', "def");        map.put('4', "ghi");        map.put('5', "jkl");        map.put('6', "mno");        map.put('7', "pqrs");        map.put('8', "tuv");        map.put('9', "wxyz");        map.put('0', " ");                StringBuilder item = new StringBuilder();        helperPhone(digits, map, res, item, 0);        return res;    }        private void helperPhone(String digits, HashMap<Character, String> map,     ArrayList<String> res, StringBuilder item, int pos) {        if (pos == digits.length()) {            res.add(item.toString());            return;        }                        String number = map.get(digits.charAt(pos));        for (int j = 0; j < number.length(); j++) {            item.append(number.charAt(j));            helperPhone(digits, map, res, item, pos + 1);            item.deleteCharAt(item.length() - 1);        }            }}