Question

write a method in java that insert First(Queue<E> s E e)that receives a queue and an elment, it inserts the element at the beginnings of queue

Answer 1

Idea

A queue data structure works on the first-in-first-out (FIFO) logic. It mainly supports 2 methods

1. push: push the element to the back of the queue.
2. pop: pop the element from the front of the queue.

Now, since we don't have any method to push or insert an element to the front of the queue, we can use the following idea: Create an empty queue. Add the new element to it's back. Then, push all the elements from the original queue to this new queue by popping them one at a time and push to the new queue. Now, we again pop all the elements from this new queue and push back to the original queue. This way, the new element is now at the front of the original queue.

The code and output are attached below. Helpful comments are also added to the code.

Note: Queue interface is implemented as a LinkedList (or PriorityQueue) in Java. But, the concept remains the same.

Code

import java.util.LinkedList;

public class QueueClass {
  public static void insert(LinkedList<Integer> q, Integer e) {
    // Create an empty queue
    LinkedList<Integer> new_q = new LinkedList<>();
    // Push new element to queue end
    new_q.add(e);
    // Pop all elements from front of original queue and push to end of new queue
    while (q.size() > 0)
      new_q.add(q.removeFirst());
    // Now, pop all elements from front of new queue and push to end original queue
    while (new_q.size() > 0)
      q.add(new_q.removeFirst());
    // Now, the original queue contains the new element at its front.
  }
  public static void main(String[] args) { 
    LinkedList<Integer> queue = new LinkedList<Integer>();
    queue.add(1);
    queue.add(2);
    queue.add(3);
    System.out.println("Elements of queue before insertion " + queue);
    insert(queue, 4);
    System.out.println("Elements of queue after insertion " + queue);
  } 
}

Output