One-liner in Python (each function in one line only). Simple!
\n    def __init__(self):\n        self.l=[]\n\n    def add(self, key: int) -> None:\n        self.l.append(key)\n\n    def remove(self, key: int) -> None:\n   
```\n    def __init__(self):\n        self.l=[]\n\n    def add(self, key: int) -> None:\n        self.l.append(key)\n\n    def remove(self, key: int) -> None:\n        self.l=[i for i in self.l if i!=key]\n\n    def contains(self, key: int) -> bool:\n        return key in self.l\n```\n\nPlease upvote if you like it!\n
['Python', 'Python3']