给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。答案可以按 任意顺序 返回。

给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。

输入：digits = "23"
输出：["ad","ae","af","bd","be","bf","cd","ce","cf"]
示例 2：

输入：digits = ""
输出：[]
示例 3：

输入：digits = "2"
输出：["a","b","c"]

暴力枚举法

class Solution:
    def letterCombinations(self, digits: str):
        ans, dct = [""], {'2': "abc", '3': "def", '4': "ghi", '5': "jkl", '6': "mno", '7': "pqrs", '8': "tuv", '9': "wxyz"}
        for c in digits:
            ans = [s1 + s2 for s1 in ans for s2 in dct[c]]
        return ans if ans[0] else []

回溯法

class Solution:
    def letterCombinations(self, digits: 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))
            else:
                digit = digits[index]
                for letter in phoneMap[digit]:
                    combination.append(letter)
                    backtrack(index + 1)
                    combination.pop()

        combination = list()
        combinations = list()
        backtrack(0)
        return combinations