Letter Combinations of a Phone Number.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"].

public class Solution {
    private char[][] map = new char[][] { { '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' } };

    public ArrayList<String> letterCombinations(String digits) {
	ArrayList<String> ret = new ArrayList<String>();
	StringBuilder sb = new StringBuilder();
	letterCombinations(digits, 0, sb, ret);
	return ret;
    }

    private void letterCombinations(String digits, int i, StringBuilder sb, ArrayList<String> ret) {
	if (i >= digits.length()) {
	    ret.add(sb.toString());
	} else {
	    int index = digits.charAt(i) - '1' - 1;
	    int size = map[index].length;
	    for (int j = 0; j < size; j++) {
		sb.append(map[index][j]);
		letterCombinations(digits, i + 1, sb, ret);
		sb.deleteCharAt(sb.length() - 1);
	    }
	}
    }
}