LC17. 电话号码的字母组合

class Solution(object):
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        res = []
        s = ""
        map = ["","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"]
        if len(digits) == 0:
            return []
        def backtrack(index,digits,s):
            if index == len(digits):
                res.append(s)
                return
            digit = int(digits[index])
            letter = map[digit]
            for i in range(len(letter)):
                s += letter[i]
                backtrack(index+1,digits,s)
                s = s[:-1]
        backtrack(0,digits,s)
        return res