Letter Combinations of a Phone Number

Source

• 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) 
given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Java

public class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> result = new ArrayList<String>();
        if(digits.length()==0) return result;
        HashMap<Character, String> map = new HashMap<Character, String>();
        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");

        dfs(digits, "", 0, map, result);
        return result;
    }
    public void dfs(String digits, String path, int index, HashMap<Character, String> map, List<String> result){
        if(index == digits.length()){
            result.add(path);
            return;
        }
        char num = digits.charAt(index);
        String letters = map.get(num);
        for(int i=0; i<letters.length(); i++){

            char c = letters.charAt(i);
            dfs(digits,  path+c, index+1, map, result);
        }
    }
}