public void reorderList(ListNode head) {
        if(head == null || head.next == null) return;
        ListNode p1 = head;
        ListNode p2 = head.next;
        while(p2 != null && p2.next != null)
        {
            p1 = p1.next;
            p2 = p2.next.next;
        }
        ListNode newHead = p1.next;
        p1.next = null;
//        LinkedList.printList("head : ",head);
//        LinkedList.printList("newHead : ",newHead);
        newHead = reverseList(newHead);
        mergeList(head,newHead);

    }