Leetcode 17 Letter Combinations of a Phone Number

class Solution:
    # @param {string} digits
    # @return {string[]}
    def letterCombinations(self, digits):
        alpha = [" ", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"] 
        nums = len(digits);res = []
        if nums==0: return []
        def Recur(num, string):
            if num==nums:
                res.append(string)
                return
            for letter in alpha[int(digits[num])]:
                Recur(num+1, string+letter)
        Recur(0,'')   
        return res