Given a digit string, return all possible letter combinations that the number could represent.

letterCombinations.java

/*
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.
*/

public class Solution {
    public ArrayList<String> letterCombinations(String digits) {
        // Start typing your Java solution below
        // DO NOT write main() function
        char[][] map = {{},{},{'a','b','c'},{'d','e','f'},
                        {'g','h','i'},{'j','k','l'},{'m','n','o'},
                        {'p','q','r','s'}, {'t','u','v'}, {'w','x','y','z'}};

        ArrayList<String> ret = new ArrayList<String>();
        if(digits.length() == 0) {
            ret.add("");
            return ret;
        }
        StringBuffer str  = new StringBuffer();
        
        combines(0, digits, str, map, ret);
        
        return ret;
    }
    
    public void combines(int start, String digits, StringBuffer str, char[][] map, ArrayList<String> ret) {
        int curNum = digits.charAt(start) - '0';
        for(int i = 0; i < map[curNum].length; i++) {
            str.append(map[curNum][i]);
            if(start == (digits.length() - 1)) {
                ret.add(str.toString());
            }
            if(start < digits.length() - 1)
                combines(start+1, digits, str, map, ret);
            // delete the last one
            str.deleteCharAt(str.length() - 1);
        }
    }
}