For each deque if you pass it in a hashset to get the unique set size, it would take time if the size is huge. Instead use a map and for each loop keep track of the occurence of deque numbers.

Here is my code take a look: ---

Scanner in = new Scanner(System.in); ArrayDeque deque = new ArrayDeque<>(); int n = in.nextInt(); int m = in.nextInt(); int max = 0;

     Map<Integer,Integer> map = new HashMap<Integer, Integer>();

     for (int i = 0; i < n; i++) {
         int num = in.nextInt();
         deque.add(num);

         if(map.containsKey(num)){
             map.put(num, map.get(num)+1);
         }
         else{
             map.put(num, 1);
         }

         if(deque.size()==m+1){
             int remove = (int) deque.remove();
             if(map.get(remove)>1){
                 map.put(remove, map.get(remove)-1);
             }
             else{
                 map.remove(remove);
             }
         }
             if(deque.size()==m){
                max = max>map.size() ? max:map.size();
             }

     }
     System.out.println(max);