17. 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"].

dfs

public class Solution {
    List<String> res = new ArrayList<>();
    String[] dict = new String[]{"","", "abc", "def", "ghi", "jkl", "mno","pqrs","tuv", "wxyz"};
    public List<String> letterCombinations(String digits) {

        combination(digits, 0, "");

        return res;
    }

    private void combination(String digits, int k, String current){
        if(k == digits.length()){
            if(!current.isEmpty())res.add(current);
            return;
        }
        String s = dict[digits.charAt(k)- '0'];
        for(Character c : s.toCharArray()){
            combination(digits, k+1, current+c);
        }
    }
}