import java.util.*;

class LinkedList {
    protected LinkedListEntry start = null;
    protected LinkedListEntry end = null;
    protected int numElements;

    public void addElement(Object obj) {
        // Make sure the object is valid
        if (obj == null)
            throw new NullPointerException();

        // Create the new entry
        LinkedListEntry newElement = new LinkedListEntry(obj);
        numElements++;

        // See if the new element is the start of the list
        if (start == null) {
            start = newElement;
            end = newElement;
        }
        else {
            end.next = newElement;
            newElement.prev = end;
            end = newElement;
        }
    }

    public void insertElementAt(Object obj, Object pos) {
        // Make sure the objects are valid
        if (obj == null || pos == null)
            throw new NullPointerException();
   
        // Make sure the position object is in the list
        LinkedListEntry posEntry = find(pos);
        if (posEntry == null)
            throw new NullPointerException();
   
        // Create the new entry
        LinkedListEntry newElement = new LinkedListEntry(obj);
        numElements++;
   
        // Link in the new entry
        newElement.next = posEntry;
        newElement.prev = posEntry.prev;
        if (posEntry == start)
            start = newElement;
        else
            posEntry.prev.next = newElement;
        posEntry.prev = newElement;
    }

    public boolean removeElement(Object obj) {
        // Make sure the object is valid
        if (obj == null)
            throw new NullPointerException();
   
        // Make sure the object is in the list
        LinkedListEntry delEntry = find(obj);
        if (delEntry == null)
            return false;
   
        // Unlink the entry
        numElements--;
        if (delEntry == start)
            start = delEntry.next;
        else
        delEntry.prev.next = delEntry.next;
        if (delEntry == end)
            end = delEntry.prev;
        else
            delEntry.next.prev = delEntry.prev;
        return true;
    }

    private LinkedListEntry find(Object obj) {
        // Make sure the list isn't empty and the object is valid
        if (isEmpty() || obj == null)
            return null;

        // Search the list for the object
        LinkedListEntry tmp = start;
        while (tmp != null) {
            if (tmp.val == obj)
                return tmp;
            tmp = tmp.next;
        }
        return null;
    }

    public boolean contains(Object obj) {
        return (find(obj) != null);
    }

    public boolean isEmpty() {
        return (start == null);
    }

    public Enumeration elements() {
        return new LinkedListEnumerator(this);
    }
}