def is_cyclic(head):
if not head:<EOL><INDENT>return False<EOL><DEDENT>runner = head<EOL>walker = head<EOL>while runner.next and runner.next.next:<EOL><INDENT>runner = runner.next.next<EOL>walker = walker.next<EOL>if runner == walker:<EOL><INDENT>return True<EOL><DEDENT><DEDENT>return False<EOL>
:type head: Node
:rtype: bool