[leetcode] 17. Letter Combinations of a Phone Number ,python實現【medium】

1. Letter Combinations of a Phone Number

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”].

圖就不上了，意思是說，手機的9宮按鍵，2對應的是abc,3對應的def，這樣。要你給出所有的可能，這在數學上叫什麼乘機來着我忘了。。。
就是用遞歸或者dfs，比如給‘234’吧，先看2，那現在應該是[‘a’,’b’,’c’]，
那再加上3呢，是不是在這個[‘a’,’b’,’c’]的基礎上，[‘a’,’b’,’c’]每一個的後面分別加上‘d’,’e’,’f’：也就是 [“ad”, “ae”, “af”, “bd”, “be”, “bf”, “cd”, “ce”, “cf”]，
然後再加4的’g”h”i’，以此類推。

下面是python實現，就是爽，主要的遞歸函數就一行。不知道c++要多少行啊。。

class Solution(object):
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        if digits == '':
            return []
        self.DigitDict=[' ','1', "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"]
        res = ['']
        for d in digits:
            res = self.letterCombBT(int(d),res)
        return res

    def letterCombBT(self, digit, oldStrList):
        return [dstr+i for i in self.DigitDict[digit] for dstr in oldStrList]