public static ListNode deleteDuplicates(ListNode head) {
        if(head == null)
        return null;

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


        while(fast!=null){
            while(fast.next!=null&&fast.val==fast.next.val){
                fast = fast.next;
            }

            if(slow.next!=fast){
             slow.next  = fast.next;
             fast = slow.next;
            }
           else{
               slow  = slow.next;
               fast  = fast.next;
           } 
        }return dummy.next;
    }