leetcode 17 Letter Combinations of a Phone Number

class Solution(object):
    
    def check(self,ret,dt,s,digits,len_digs):
        s_len = len(s)
        if s_len == len_digs:
            ret.append(s)
            return
        
        for v in dt[digits[s_len]]:
            s = s + v
            self.check(ret,dt,s,digits,len_digs)
            s = s[:-1]
            
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        dt={'2':"abc",'3':"def",'4':"ghi",'5':"jkl",'6':"mno",'7':"pqrs",'8':"tuv",'9':"wxyz"}
        s=""
        len_digs=len(digits)
        ret=[]
        if len_digs > 0:
            self.check(ret,dt,s,digits,len_digs)
        return ret