class  {
    public ListNode deleteDuplicates(ListNode head) {
        if (head == null) {
            return head;
        }
        ListNode pre = head;
        ListNode now = head;
        ListNode next = head.next;
        while (next != null) {
            if (now.val != next.val) {
                pre = now;
                now = next;
                next = now.next;
            } else {
                while (next != null) {
                    if (now.val == next.val) {
                        now = next;
                        next = now.next;
                    } else {
                        break;
                    }
                }
                if (next != null) {
                    if (head.val == now.val) {
                        head = next;
                        pre = head;
                        now = head;
                        next = head.next;
                    } else {
                        pre.next = next;
                        now = pre.next;
                        next = now.next;
                    }

                } else {
                    if (head.val == now.val) {
                        head = null;
                    } else {
                        pre.next = next;
                    }

                }
            }
        }
        return head;
    }
}