Folding a linked list using a Stack

Question

Here is my program to fold a linked list using a Stack:

    public Node middle(Node head) {
            Node slow = head;
            Node fast = head;
            while (fast != null && fast.next != null) {
                fast = fast.next.next;
                slow = slow.next;
            }
            return slow;
        }

        public Node foldList(Node head){
            Node mid = middle(head);
            Node f = mid.next;
            Stack<Node> stacks = new Stack<>();
            if (head == null) return head;
            while (f != null){
                stacks.push(f);
                f = f.next;
            }
            Node temp = head;
            Node forv = head.next;
            while(!stacks.isEmpty()) {
                temp.next = stacks.pop();
                temp = temp.next;
                temp.next = forv;
                temp = temp.next;
                forv = forv.next;
            }
            return head;
        }

Here is the code of the middle() and foldList() methods. When I run it it gets stuck in an infinite loop. Can anybody help me find out why this is happening?