public interface Queue211<E> {
  boolean add(E e); // add e to the end of the queue. May throw an IllegalStateException
  E element(); // returns the front of the queue w/o removing
  boolean offer(E e); // inserts e to into the queue.
  E peek(); // returns the front of the queue or null.
  E poll(); // removes and returns the front of the queue or null.
  E remove(); // removes and returns the front of the queue, may throw an IllegalStateException.
}

public class LinkedQueue<E> implements Queue211<E> {
  private LinkedNode<E> front;
  private LinkedNode<E> rear;
  private int size;
  ...
  public E poll() {
    // your code here.
  }
  ...
}