17_Letter Combinations of a Phone Number

class Solution:
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        my_dict = {'2':'abc', '3':'def', '4':'ghi', '5':'jkl', '6':'mno', 
        '7':'pqrs', '8':'tuv', '9':'wxyz'}
        if len(digits) == 0:
            return []
        elif len(digits) == 1:
            return list(my_dict[digits[0]])
        prev = self.letterCombinations(digits[:-1])
        additional = my_dict[digits[-1]]
        return [s + c for s in prev for c in additional]
s = Solution()
print(s.letterCombinations('23'))