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.

Solution:

public class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> res = new ArrayList<String>();
        List<List<Character>> map = getMap();
        char[] comb = new char[digits.length()];
        int[] num = new int[digits.length()];
        for (int i=0;i<digits.length();i++) num[i] = (digits.charAt(i)-'0')-2;
        int[] curPos = new int[digits.length()];
        Arrays.fill(curPos,-1);
        int level = 0;
        while (level!=-1){
            if (level>=digits.length()){
                for (int i=0;i<digits.length();i++)
                    comb[i] = map.get(num[i]).get(curPos[i]);
                String temp = new String(comb);
                res.add(temp);
                level--;
                continue;
            }
            int val = curPos[level];
            if (val+1>=map.get(num[level]).size()){
                curPos[level]=-1;
                level--;
                continue;
            } else {
                curPos[level]++;
                level++;
            }
        }

        return res;        
    }


    public List<List<Character>> getMap(){
        int[] val = new int[]{3,3,3,3,3,4,3,4};
        List<List<Character>> map = new ArrayList<List<Character>>();
        int base = -1;
        for (int i=0;i<val.length;i++){
            List<Character> list = new ArrayList<Character>();
            for (int j=0;j<val[i];j++){
                base++;
                list.add((char)('a'+base));
            }
            map.add(list);
        }
        return map;
    }
          
}