[LeetCode] Letter Combinations of a Phone Number

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.

Solution: There are many ways to implement this. The one I enjoy most is by recursion. 

public class Solution {
    public List<String> letterCombinations(String digits) {
        ArrayList<String> list = new ArrayList<String>();
        if (digits.length() == 0) return list;
        String[] strs = new String[]{
            "", "abc", "def", 
            "ghi", "jkl", "mno", 
            "pqrs", "tuv", "wxyz"
        };
        int digit = digits.charAt(0) - '0';
        ArrayList<String> l = new ArrayList<String>(
            letterCombinations(digits.substring(1))
        );
        String str = strs[digit - 1];
        for (int i = 0; i < str.length(); i++) {
            char ch = str.charAt(i);
            if (l.size() == 0) {
                list.add(ch + "");
                continue;
            }
            for (String s : l) {
                list.add(ch + s);
            }      
        }
        return list;
    }
}