Phone Number

Given a digit string, return all possible letter combinations that the number could represent.

Example

Given 23

Return ["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.

Solution

public class Solution {
    /**
     * @param digits A digital string
     * @return all posible letter combinations
     */
    public ArrayList<String> letterCombinations(String digits) {
        ArrayList<String> res = new ArrayList<>();
        if(digits == null ||digits.length() == 0)
            return res;
        HashMap<Character, String> dict = new HashMap<>();
        dict.put('2',"abc");dict.put('3',"def");dict.put('4',"ghi");dict.put('5',"jkl");
        dict.put('6',"mno");dict.put('7',"pqrs");dict.put('8',"tuv");dict.put('9',"wxyz");

        helper(dict, res, "", digits, 0);
        return res;
    }

    private void helper(HashMap<Character, String> dict, ArrayList<String> res, String str, String digits, int idx) {
        if(idx == digits.length()) {
            res.add(new String(str));
            return;
        }
        String letters = dict.get(digits.charAt(idx));
        for(int i = 0; i < letters.length(); i++) {
            str += letters.charAt(i);
            helper(dict, res, str, digits, idx + 1);
            str = str.substring(0, idx);
        }
    }
}