import java.util.*;

public class MyQueueList implements MyQueue {

    private ListNode front; // read from here
    private ListNode back;  // write to here

    public MyQueueList() {
        front = null;
    }
  
    public boolean isEmpty() {
        return front == null;
    }
  
    // join the back
    public void enqueue(Object item) {
        if (front == null) {
            front = back = new ListNode(item, null);
        } else {
            back = back.next = new ListNode(item, null);
        }
    }

    // leave the front
    public Object dequeue() {
        if (front == null) {
            throw new NoSuchElementException();
        } else {
            Object item = front.data;
            front = front.next;
            return item; 
        }
    }

    public Enumeration elements() {
        // put your code here
    }

}