Big O week 3 day 6 p1

    public static Node firstNode(Node head) {
        Node slow = head, fast = head;
        // 1->2->3->4
        while(fast.next != null && fast.next.next != null){
            slow = slow.next;
            fast = fast.next.next;
            if(slow == fast){
                break;
            }
        }
        if(fast.next == null || fast.next.next == null){
            return null;
        }
        Node slow2 = head;
        while(slow.next != null){
            if(slow == slow2) return slow;
            slow = slow.next;
            slow2 = slow2.next;
        }
        return null;
    }