题目：

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

代码：

每日一题哦，今天做到这个题目，仔细一看，就会找到数字对应的字符串，然后把字符串组合起来，返回一个列表。

想想就不应该很复杂，可是，还是做了一个小时，唉，看来必须每天坚持练习练习！

逻辑很简单，就是用一个列表保存每次两个字符串相加的结果，从digits的第一个元素开始，不断和下一次相加，一直加到digits最后一位元素：

def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        letter_dic = {2:"abc",3:"def",4:"ghi",5:"jkl",6:"mno",7:"pqrs",8:"tuv",9:"wxyz"}
        if len(digits)==0:return []
        res_list = [list(letter_dic[int(digits[0])])]
        digits = digits[1:]
        while len(digits) !=0:                
            res_list.append(self.str_connect(res_list[-1],list(letter_dic[int(digits[0])])))       
            digits = digits[1:]
            #print (res_list)
        return res_list[-1]
    
    def str_connect(self,str_ori,str_add):
        res = []
        for i in range(0,len(str_ori)):
            for j in str_add:
                res.append(str_ori[i]+j)
        return res