Leetcode 17 Letter Combinations of a Phone Number

class Solution:
    # @param {string} digits
    # @return {string[]}
    def letterCombinations(self, digits):
        Dict = {"0":'', "1":'', "2": 'abc',"3": 'def', "4": 'ghi',  
               "5": 'jkl',"6": 'mno',"7": 'pqrs',"8": 'tuv',"9": 'wxyz'}
        Length = len(digits);result = []
        def rescur(num, String):
            if num >= Length:
                result.append(String)
                return
            for letter in Dict[digits[num]]:
                rescur(num+1,String+letter)
        if Length>0: rescur(0, '') 
        else: return []
        return result