from typing import List


class Solution:
    def letterCombinations(self, digits: str) -> List[str]:
        if len(digits) == 0:
            return []

        options = {
            "2": ("a", "b", "c"),
            "3": ("d", "e", "f"),
            "4": ("g", "h", "i"),
            "5": ("j", "k", "l"),
            "6": ("m", "n", "o"),
            "7": ("p", "q", "r", "s"),
            "8": ("t", "u", "v"),
            "9": ("w", "x", "y", "z"),
        }

        pools = (options[digit] for digit in digits)
        result = [""]
        for pool in pools:
            result = [x + y for x in result for y in pool]
        return result