[Leetcode][python]Letter Combinations of a Phone Number/电话号码的字母组合

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

        def dfs(num, string, res):
            print string, res
            if num == length:
                res.append(string)
                return
            for letter in dict[digits[num]]:
                print letter
                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)
        if length == 0:
            return []
        dfs(0, '', res)
        return res