public static ListNode deleteDuplicates2(ListNode head) {//pass
        if (head == null || head.next == null) {
            return head;
        }
        ListNode p = head;
        while (p != null && p.next != null) {
            while (p.next != null && p.next.val == p.val) {
                p.next = p.next.next;
            }
            p = p.next;
        }
        return head;
    }