public class Solution {
        public ListNode deleteDuplication(ListNode head)
        {
            if(head==null||head.next==null)
                return head;
           ListNode vHead = new ListNode(-1);
            vHead.next = head;
            ListNode p = vHead;
            ListNode q = p.next;
            while (p.next!=null) {

                while (q!=null && p.next.val == q.val) q = q.next;

                if ( p.next.next == q) p = p.next;
                else p.next = q;
                q=p.next;
            }

            return vHead.next;
        }
    }