17. Letter Combinations of a Pho

【Description】
Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.

Example:

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

【Idea】
base解法的逻辑性要求不是很高，主要是代码实现的两点trick：

1. 数字到字符的映射，定义list： ['', '', 'abc', 'def', 'ghi', 'jkl', 'mno', 'pqrs', 'tuv', 'wxyz'] 将数字以下标形式表示；
2. result = [ori_str+i for i in digit_str[int(temp_num)] for ori_str in result] 实现嵌套循环。

【Solution】

class Solution:
    def letterCombinations(self, digits):
        if digits == "":
            return []
        digit_str = ['', '', 'abc', 'def', 'ghi', 'jkl', 'mno', 'pqrs', 'tuv', 'wxyz']
        result = ['']
        for temp_num in digits:
            result = [ori_str+i for i in digit_str[int(temp_num)] for ori_str in result]
        return result