public ListNode insertNode(ListNode head, int val) { 
        ListNode node = new ListNode(val);
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        head = dummy;
        while(head.next!=null&&head.next.val<=val){
                head = head.next;
        }
        node.next = head.next;
        head.next = node;
        return dummy.next;
    }