[leetcode]Letter Combinations of a Phone Number @ Python

原題地址：https://oj.leetcode.com/problems/letter-combinations-of-a-phone-number/

題意：

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.

解題思路：窮舉所有可能的字符串使用dfs來解決。

代碼：

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