17. Letter Combinations of a Phone Number Leetcode Python

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.

这道题用的是BFS的方法，枚举每个组合。 复杂度是NP Hard 随着数字的增加，subarray增加。

代码如下：

class Solution:
    # @return a list of strings, [s1, s2]
    def letterCombinations(self, digits):
        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']}
        def dfs(num,res,substr):
            if num==len(digits):
                res.append(substr)
                return
            for letter in dict[digits[num]]:
                dfs(num+1,res,substr+letter)
        solution=[]
        dfs(0,solution,"")
        return solution