:type head: Node
    :rtype: bool

def is_cyclic(head):
    """
    :type head: Node
    :rtype: bool
    """
    if not head:
        return False
    runner = head
    walker = head
    while runner.next and runner.next.next:
        runner = runner.next.next
        walker = walker.next
        if runner == walker:
            return True
    return False