HackerRank-Solutions/Java/Data Structures/Java Dequeue/Solution.java

//     Author: Rodney Shaghoulian
//     Github: github.com/rshaghoulian
// HackerRank: hackerrank.com/rshaghoulian

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());
        }
        
        scan.close();
        System.out.println(max);
    }
}