原题地址：https://oj.leetcode.com/problems/letter-combinations-of-a-phone-number/
题意：
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.
解题思路：穷举所有可能的字符串使用dfs来解决。
代码：
class Solution: # @return a list of strings, [s1, s2] def letterCombinations(self, digits): def dfs(num, string, res): if num == length: res.append(string) return for letter in dict[digits[num]]: dfs(num+1, string+letter, res) dict = {‘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‘] } res = [] length = len(digits) dfs(0, ‘‘, res) return res
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