Leetcode, Letter Combinations of a Phone Number

lcopm.py

from typing import List

class Solution:

    def __init__(self):
        self.letter_dict = {"2": "abc", "3": "def", "4": "ghi", "5": "jkl", "6": "mno", "7": "pqrs", "8": "tuv", "9": "wxyz"}


    def letterCombinations(self, digits: str) -> List[str]:
        if digits == "" or len(digits) == 0:
            return digits
        if len(digits) == 1:
            return list(self.letter_dict[digits[0]])
        if len(digits) == 2:
            result = []
            for top in self.letter_dict[digits[0]]:
                for bottom in self.letter_dict[digits[1]]:
                    result.append(top + bottom)
            return result

        result = []
        for top in self.letter_dict[digits[0]]:
            for bottom in self.letterCombinations(digits[1:]):
                result.append(top + bottom)
        return result