python
from typing import List
class Solution:
    def letterCombinations(self, digits: str) -> List[str]:
        if not digits:
            return list()
        phoneMap = {
            "2": "abc",
            "3": "def",
            "4": "ghi",
            "5": "jkl",
            "6": "mno",
            "7": "pqrs",
            "8": "tuv",
            "9": "wxyz",
        }
        def backtrack(index: int):
            if index == len(digits):
                combinations.append("".join(combination))
                return
            digit = digits[index]
            for letter in phoneMap[digit]:
                combination.append(letter)
                backtrack(index + 1)
                combination.pop()
        combination = list()
        combinations = list()
        backtrack(0)
        return combinations
print(Solution().letterCombinations("234"))