Leetcode-17 - Simple

class Solution {

    private static final Map<Character, List<Character>> MAPPING = Map.ofEntries(
        Map.entry('1', List.of()),
        Map.entry('2', List.of('a', 'b', 'c')),
        Map.entry('3', List.of('d', 'e', 'f')),
        Map.entry('4', List.of('g', 'h', 'i')),
        Map.entry('5', List.of('j', 'k', 'l')),
        Map.entry('6', List.of('m', 'n', 'o')),
        Map.entry('7', List.of('p', 'q', 'r', 's')),
        Map.entry('8', List.of('t', 'u', 'v')),
        Map.entry('9', List.of('w', 'x', 'y', 'z'))
    );


    private List<String> result = null;
    private void compute(String digits, int index, StringBuilder stringBuilder) {
        if (index == digits.length()) {
            result.add(stringBuilder.toString());
            return;
        }

        MAPPING.get(digits.charAt(index)).forEach(value -> {
            stringBuilder.append(value);
            compute(digits, index + 1, stringBuilder);
            stringBuilder.setLength(stringBuilder.length() - 1);
        });
    }

    public List<String> letterCombinations(String digits) {
        if (digits.isEmpty()) {
            return List.of();
        }
        result = new ArrayList<>();
        compute(digits, 0, new StringBuilder());
        return result;
    }

}