LeetCode-algorithms 17. Letter Combinations of a Phone Number

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"].

思路：

这题在理解上并不困难，而实际上麻烦在于实现上。因为不能确定输入的数字的个数，因此不能使用for循环，本体实际上属于一种排列组合，可以构建一颗树，每一层则表示一个数字所能表示的字母。那么只要遍历整一棵树即可得到答案。

代码：

class Solution(object):    dict = {        '1':[''],        '2':['a','b','c'],        '3':['d','e','f'],        '4':['g','h','i'],        '5':['j','k','l'],        '6':['m','n','o'],        '7':['p','q','r','s'],        '8':['t','u','v'],        '9':['w','x','y','z'],        '0':[' ']        }    def letterCombinations(self, digits):        """        :type digits: str        :rtype: List[str]        """        result = []        begin = []        for d in digits:            begin.append(self.dict[d])        dfs = [['',begin]]        while len(dfs) != 0:            state = dfs.pop()            if len(state[1]) == 1:                for c in state[1][0]:                    result.append(state[0]+c)            elif len(state[1])!= 0:                for c in state[1][0]:                    dfs.append([state[0]+c,state[1][1:]])        return result

结果：