17. Letter Combinations of a Phone Number

class Solution:  
    def letterCombinations(self, digits: str) -> List[str]:
        if len(digits) == 0:
            return []
        m = {
            "2": "abc",
            "3": "def",
            "4": "ghi",
            "5": "jkl",
            "6": "mno",
            "7": "prqs",
            "8": "tuv",
            "9": "wxyz"
        }

        results = [""]
        for digit in digits:
            results_new = []
            for pre in results:
                for c in m[digit]:
                    results_new.append(pre+c)
            results = results_new

        return results