leetcode: Letter Combinations of a Phone number

class Solution {
	public List<String> letterCombinations(String digits) {
		List<String> answer = new ArrayList<String>();

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

		for (char ch : digits.toCharArray()) {
			int number = ch - '0';
			answer = addCharacter(numberToString.get(number), answer);
		}

		return answer;
	}

	public List<String> addCharacter(String str, List<String> answer) {
		List<String> nextAnswer = new ArrayList<String>();

		if (answer.isEmpty()) {
			return str.chars().mapToObj(ch -> Character.toString(ch)).collect(Collectors.toList());
		}

		for (int i = 0; i < str.length(); i++) {
			String lastString = str.substring(i, i + 1);

			for (String substring : answer) {
				nextAnswer.add(substring + lastString);
			}
		}

		return nextAnswer;
	}
}