17.txt

class Solution {

    public ArrayList<String> output = new ArrayList<>();
    HashMap<String, String> numbers = new HashMap<>() {
        {
            put("2", "abc");
            put("3", "def");
            put("4", "ghi");
            put("5", "jkl");
            put("6", "mno");
            put("7", "pqrs");
            put("8", "tuv");
            put("9", "wxyz");
        }
    };

    public void Combinations(String combination, String digits) {
        if (digits.length() == 0) {
            output.add(combination);
        } else {
            String digit = digits.substring(0, 1);
            String letters = numbers.get(digit);
            for (int i = 0; i < letters.length(); i++) {
                String letter = letters.substring(i, i + 1);
                Combinations(combination + letter, digits.substring(1));
            }
        }
    }

    public List<String> letterCombinations(String digits) {
        if (digits.length() == 0) {
            return new ArrayList<>();
        } else {
            Solution s = new Solution();
            s.Combinations("", digits);
            return s.output;
        }
    }
}