LeetCode. 17. Letter Combinations of a Phone Number

class Solution {
    int len;
    public List<String> letterCombinations(String digits) {
        if(digits.length() == 0) return new ArrayList<String>();
        ArrayList<String> answer = new ArrayList<String>();
        ArrayList<ArrayList<Character>> dialNum = new ArrayList<> ();
        StringBuilder sb = new StringBuilder();
        int curIdx = 0, number;
        for(int i = 2; i < 10; i++) {
            if(i == 7 || i == 9) number = 4;
            else number = 3;
            ArrayList<Character> temp = new ArrayList<>();
            for(int j = 0; j < number; j++) temp.add((char)('a'+ curIdx++));
            dialNum.add(temp);
        }
        len = dialNum.size();
        dfs(digits, 0, dialNum, sb, answer);
        return answer;
    }

    void dfs(String digits, int index, ArrayList<ArrayList<Character>> dialNum, StringBuilder sb, ArrayList<String> answer) {
        if(index == digits.length()) {
            answer.add(sb.toString());
            return;
        }
        for(char c: dialNum.get(digits.charAt(index) - '2')) {
            sb.append(c);
            dfs(digits, index + 1, dialNum,  sb, answer);
            sb.setLength(Math.max(0, sb.length() - 1));
        }
    }
}