void build(String digits, int lvl) {
		if (lvl == digits.length()) {
			String s = sb.toString();
			ret.add(s);
			return;
		} else {
			char c = digits.charAt(lvl);
			int i = Integer.valueOf(c) - '0';
			if (i == 0 || (i <= 9 && i >= 2)) {
				List<Character> list = digitals.get(i);
				for (Character c1 : list) {
					sb.append(c1);
					build(digits, lvl + 1);
					sb.deleteCharAt(sb.length() - 1);
				}
			} else {
				return;
			}
		}
	}