*的字母组合

题目描述：

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。

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。

示例:

输入："23"
输出：["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

class Solution(object):
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        if len(digits) == 0:
            return []
            
        d = {1: "", 2: "abc", 3: "def", 4: "ghi", 5: "jkl", 6: "mno", 7: "pqrs", 8: "tuv", 9: "wxyz"}
        
        def dfs(digits, index, path, res, d):
            if index == len(digits):
                res.append("".join(path))
                return 
        
            digit = int(digits[index])
            for c in d.get(digit, []):
                path.append(c)
                dfs(digits, index + 1, path, res, d)
                path.pop()
        
        res = []
        dfs(digits, 0, [], res, d)
        return res
                

class Solution:
    def letterCombinations(self, digits: str):
        keys = {'2':'abc','3':'def','4':'ghi','5':'jkl','6':'mno','7':'pqrs','8':'tuv','9':'wxyz'}
        words= [keys[_] for _ in digits if _ in keys]
        if len(words) == 0:
            return []
        else:
            ans = [_ for _ in words[0]]
        for i in range(1,len(words)):
            temp = ans
            for j in range(len(words[i])):
                a = [(_ + words[i][j]) for _ in temp]
                if j == 0:
                    ans = a
                else:
                    ans += a

        return ans