Reverse_First_K_elements_of_Queue.java

// { Driver Code Starts
// Initial Template for Java

import java.util.*;

class ModifyQueue {
  public static void main(String[] args) {

    // Taking input using class Scanner
    Scanner sc = new Scanner(System.in);

    // Taking total number of testcases
    int t = sc.nextInt();

    while (t-- > 0) {
      // Taking count of total number of elements
      int n = sc.nextInt();

      // Taking count of total elements
      // that need to be reversed
      int k = sc.nextInt();

      // Creating a Queue
      Queue<Integer> q = new LinkedList<>();

      // adding all the elements to the Queue
      while (n-- > 0) {
        q.add((int) sc.nextInt());
      }

      // Creating an object of class GfG
      GfG g = new GfG();

      // calling modifyQueue of class GfG
      // and passing Queue and k as arguments
      // and storing the reuslt in a new Queue
      Queue<Integer> ans = g.modifyQueue(q, k);

      // Printing all the elements from the
      // new Queue and polling them out
      while (!ans.isEmpty()) {
        int a = ans.peek();
        ans.poll();
        System.out.print(a + " ");
      }
      System.out.println();
    }
  }
}

class GfG {
  // Function to reverse first k elements of a queue.
  public Queue<Integer> modifyQueue(Queue<Integer> q, int k) {
    Stack<Integer> s = new Stack<Integer>();
    int n = q.size();
    for (int i = 0; i < k; i++) {
      s.push(q.poll());
    }
    while (!s.isEmpty()) {
      q.add(s.pop());
    }
    int rem = n - k;
    while (rem-- > 0) {
      q.add(q.poll());
    }
    return q;
  }
}