Solution.java

// github.com/RodneyShag

import java.util.Scanner;
import java.util.ArrayDeque;
import java.util.HashMap;

// Why not use just a HashMap instead of both a HashMap and an ArrayDeque? 
// Well, an ArrayDeque helps keep the ordering of elements. Although the 
// elements are also in our HashMap, they is no ordering to the elements 
// in the HashMap since it's just a set.
    
//  Time Complexity: O(n)
// Space Complexity: O(n)

public class test {
    public static void main(String[] args) {
        HashMap<Integer, Integer> map = new HashMap();
        ArrayDeque<Integer> deque     = new ArrayDeque();
        
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        int m = scan.nextInt();
        int max = 0;
        
        for (int i = 0; i < n; i++) {
            /* Remove old value (if necessary) */
            if (i >= m) {
                int old = deque.removeFirst();
                if (map.get(old) == 1) {
                    map.remove(old);
                } else {
                    map.merge(old, -1, Integer::sum);
                }
            }
            
            /* Add new value */
            int num = scan.nextInt();
            deque.addLast(num);
            map.merge(num, 1, Integer::sum);
            
            max = Math.max(max, map.size());
            
            /* If all integers are unique, we have found our largest
               possible answer, so we can break out of loop */
            if (max == m) {
                break;
            }
        }
        
        scan.close();
        System.out.println(max);
    }
}

// Discuss on HackerRank: https://www.hackerrank.com/challenges/java-dequeue/forum/comments/254876