Letter Combinations of a Phone Number

class Solution(object):    def letterCombinations(self, digits):        res = []        if not digits:            return res        d = {            "2":"abc",             "3": "def",            "4":"ghi",             "5": "jkl",             "6":"mno",             "7": "pqrs",             "8":"tuv",             "9": "wxyz",            }        def backtrack(i, curStr):            if len(curStr) == len(digits):                res.append(curStr)                return            for c in d[digits[i]]:                backtrack(i + 1, curStr + c)        backtrack(0, "")        return res