九宫格键盘输入

Letter Combinations of a Phone Number

• 给定一个数字字符串，返回数字可能代表的所有可能的字母组合。

• 数字到字母的映射（就像九宫格电话按钮一样）如下图。

• 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.

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

example 1

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

思路

1. 以23为例，2代表abc，只需要将其转换成lists = [a,b,c]，然后处理3，3为def，将这三个字符分别加到s的每一个元素中，得到["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"]。重复此过程即可。

2. 上述思路可以用循环完成，同时python提供的reduce函数和生成式特性能够精炼地表述，下面给出两种代码。

代码

class Solution(object):
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        from functools import reduce #python3.x
        if not digits:
            return []
        nums = '0 1 abc def ghi jkl mno pqrs tuv wxyz'.split(' ')
        return reduce(lambda last, d: [x + y for x in last for y in nums[int(d)]], digits, [''])  # 第三个参数是initial



    


    #使用循环完成
    def letterCombinations_old(self, digits):
        if not digits:
            return []
        nums = '0 1 abc def ghi jkl mno pqrs tuv wxyz'.split(' ')
        ret = [_ for _ in nums[int(digits[0])]]
        digits = digits[1:]
        while digits:
            ret = [x + y for x in ret for y in nums[int(digits[0])]]
            digits = digits[1:]
        return ret

本题以及其它leetcode题目代码github地址: github地址