(head)
return False
:type head: Node
    :rtype: bool
:type head: Node
    :rtype: bool
: 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
def is_cyclic ( head ) : 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