Leetcode: Letter Combinations of a Phone Number

Question

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.

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Solution

class Solution(object):
    def letterCombinations(self, digits):
        """ :type digits: str :rtype: List[str] """

        self.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 = []
        if digits=='' or len(digits)==0:
            return res
        self.helper(digits, 0,'',res)
        return res

    def helper(self, digits, num, string, res):
        if num==len(digits):
            res.append(string)
            return

        for letter in self.dict[digits[num]]:
            self.helper(digits, num+1, string+letter, res)