public ListNode removeDuplicates(ListNode head) { 
        HashSet<Integer> set = new HashSet<Integer>();

        ListNode dummy = new ListNode(0);
        dummy.next = head;
        head = dummy;

        while(head.next!=null){
            if(set.contains(head.next.val)){
                head.next = head.next.next;
            }
            else{
                set.add(head.next.val);
                head = head.next;
            }
        }
        return dummy.next;
    }