letter combinations of a phone number

class (object):
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        if not digits:
            return []
        int2chr = {1: "*", 2: "abc", 3: "def", 4: "ghi", 5: "jkl", 6: "mno", 7: "pqrs", 8: "tuv", 9: "wxyz"}
        res = []
        self.dfs(digits, 0, [], int2chr, res)
        return res
        
    def dfs(self, digits, pos, cur, int2chr, res):
        if pos == len(digits):
            res.append("".join(cur))
            return
        l = int2chr[int(digits[pos])]
        for i in range(len(l)):
            cur.append(l[i])
            self.dfs(digits, pos+1, cur, int2chr, res)
            cur.pop()