(head)
return runner
:type head: Node
    :rtype: Node
:type head: Node
    :rtype: Node
: type head : Node : rtype : Node
def firstCyclicNode(head):
    """
    :type head: Node
    :rtype: Node
    """
    runner = walker = head
    while runner and runner.next:
        runner = runner.next.next
        walker = walker.next
        if runner is walker:
            break

    if runner is None or runner.next is None:
        return None

    walker = head
    while runner is not walker:
        runner, walker = runner.next, walker.next
    return runner
def firstCyclicNode ( head ) : runner = walker = head while runner and runner . next : runner = runner . next . next walker = walker . next if runner is walker : break if runner is None or runner . next is None : return None walker = head while runner is not walker : runner , walker = runner . next , walker . next return runner