# 1.3.12 practice
    @classmethod
    def copy(cls, old_stack: Stack) -> None:
        """
        Copy existing stack and return new stack.
        Args:
            cls: Stack class
            stack (Stack): existing stack
        Returns:
            Stack: new stack
        >>> s = Stack()
        >>> s.push(1)
        >>> s.push(2)
        >>> s.push(3)
        >>> s_copy = Stack.copy(s)
        >>> s_copy._first.val
        3
        >>> s_copy.size()
        3
        >>> for item in s_copy:
        ...     print(item)
        ...
        3
        2
        1
        """
        s_tmp = cls()
        s_copy = cls()
        if isinstance(old_stack, Stack) and not old_stack.is_empty():
            for item in old_stack:
                s_tmp.push(item)
            for item in s_tmp:
                s_copy.push(item)
        return s_copy