判断链表是否有环

方法一：set集合法，数据节点存在则说明有环

def create_circle_linklist():
    a = Node(1)
    b = Node(2)
    c = Node(3)
    d = Node(4)
    a.next = b
    b.next = c
    c.next = d
    d.next = b
    return a


def check_circle(head):
    a = set()
    while head:
        if head not in a:
            a.add(head)
        else:
            return False
        head = head.next
    return True


if __name__ == '__main__':
    print(check_circle(create_circle_linklist()))