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