天池编程大赛周赛-Character deletion

class Solution:
    """
    @param str: The first string given
    @param sub: The given second string
    @return: Returns the deleted string
    """
    def CharacterDeletion(self, str, sub):
        # write your code here
        result = []
        sub = set(sub)
        for s in str:
            if s in sub:
                pass
            else:
                result.append(s)
        return "".join(result)