private void search(ArrayList<String> ans, String digits, int index,
                char[] letters) {
            if (index == digits.length()) {
                ans.add(new String(letters));
                return;
            }

            String options = dict[digits.charAt(index) - '0'];
            for (int i = 0; i < options.length(); i++) {
                letters[index] = options.charAt(i);
                search(ans, digits, index + 1, letters);
            }
        }