Leetcode-17 - Fast

class Solution {
    private static final char[][] FAST_MAP = initChar();
    private static char[][] initChar() {
        char[][] result = new char[10][];
        result[0] = new char[]{};
        result[1] = new char[]{};
        result[2] = new char[]{'a', 'b', 'c'};
        result[3] = new char[]{'d', 'e', 'f'};
        result[4] = new char[]{'g', 'h', 'i'};
        result[5] = new char[]{'j', 'k', 'l'};
        result[6] = new char[]{'m', 'n', 'o'};
        result[7] = new char[]{'p', 'q', 'r', 's'};
        result[8] = new char[]{'t', 'u', 'v'};
        result[9] = new char[]{'w', 'x', 'y', 'z'};
        return result;
    }


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

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

        for (char value : FAST_MAP[digits.charAt(index) - '0']) {
            stringBuilder.append(value);
            compute(digits, index + 1, stringBuilder);
            stringBuilder.setLength(stringBuilder.length() - 1);
        };
    }

}