leetcode 17. 电话号码的字母组合

没啥思路直接写就完了···

dict_ = {"2":"abc","3":"def","4":"ghi","5":"jkl","6":"mno","7":"pqrs","8":"tuv","9":"wxyz"}
        res = []
        for i in digits:
            if len(res)==0:
                res.extend([j for j in dict_[i]])
            else:
                res = [w+j for j in dict_[i] for w in res]
        return res