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:
Trivial implementation question.
Tips: 7 and 9 have four letters.

public class Solution {
    public ArrayList letterCombinations(String digits) {
        String[] map = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};  // 2 -> 0
        
        ArrayList prev = new ArrayList();
        prev.add("");
        
        for(int i = 0; i < digits.length(); i++){
            ArrayList cur = new ArrayList();
            
            // all combinations
            char[] tmp = map[digits.charAt(i) - '0' - 2].toCharArray();
            for(String str : prev){
                for(char c : tmp){
                    cur.add(str+c);
                }
            }
            
            prev = cur;
        }
        
        return prev;
    }
}