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.

Brute force method can be accepted by leetcode online judge.

public class Solution {
    public ArrayList<String> letterCombinations(String digits) {
        String[] keyboard = new String[]{"","","abc","def","ghi","jkl","mno",
												 "pqrs","tuv","wxyz"};
		ArrayList<String> result = new ArrayList<String>();
		result.add("");
		for(int i = 0; i < digits.length(); ++i){
			int k = digits.charAt(i) - 48;
			if(k != 0 && k != 1){
				ArrayList<String> temp = new ArrayList<String>();
				while(!result.isEmpty()){
					String s = result.remove(0);
					for(int j = 0; j < keyboard[k].length(); ++j){
						StringBuffer stb = new StringBuffer(s);
						stb.append(keyboard[k].charAt(j));
						temp.add(stb.toString());
					}
				}
				result = temp;
			}
		}
		return result;
    }
}