import java.util.NoSuchElementException;

public class OrderedList implements OrderedStructure {

    // Implementation of the doubly linked nodes (nested-class)

    private static class Node {

	private Comparable value;

	private Node previous;
	private Node next;

	private Node ( Comparable value, Node previous, Node next ) {
	    this.value = value;
	    this.previous = previous;
	    this.next = next;
	}
    }

    // Instance variables

    private Node head;
    private Node tail;

    // Representation of the empty list.

    public OrderedList() {
        head = null;
	tail = null;
    }

    // Calculates the size of the list

    public int size() {
	Node p = head;
	int count = 0;
	while ( p!=null ) {
	    p = p.next;
	    count++;
	}
	return count;
    }

    // Adding an element while preserving the order

    public boolean add( Comparable o ) {

	if ( o == null )
	    throw new IllegalArgumentException( "null" );

	if ( head == null ) { // special case: empty list

	    head = tail = new Node( o, null, null );

	} else if ( head.value.compareTo(o) >= 0 ) { // special case: add before first node

	    head = new Node( o, null, head );

	    head.next.previous = head;

	} else { 
	    
	    Node p = head;

	    // i)  there is at least one node
	    // ii) o is greater than p

	    while ( p.next != null && p.next.value.compareTo( o ) < 0 ) {
		p = p.next;
	    }

	    if ( p.next == null ) { // adding at the end of the list

		tail.next = new Node( o, tail, null );

		tail = tail.next;

	    } else { // intermediate position

		Node q = p.next; // the node that follows

		p.next = new Node( o, p, q );

		q.previous = p.next;

	    }
	}

	return true;
    }

    // This implementation does not call size() to determine
    // if pos is valid; therefore, the list is only traversed
    // once.

    public Object get( int pos ) {

	if (pos < 0) 
	    throw new IndexOutOfBoundsException( Integer.toString( pos ) );

	Node p = head;
	
	for ( int i=0; i<pos; i++ )
	    if ( p == null )
		throw new IndexOutOfBoundsException( Integer.toString( pos ) );
	    else
		p = p.next;

	return p.value;
    }

    public void remove( int pos ) {

	if ( pos < 0 ) 
	    throw new IndexOutOfBoundsException( Integer.toString( pos ) );

	Node p = head;
	
	if ( pos == 0 ) {

	    if ( head == null ) {
		throw new IndexOutOfBoundsException( Integer.toString( pos ) );
	    }

	    head = head.next;

	    if ( head == null )
		tail = null; // this was the last element
	    else
		head.previous = null;

	    p.value = null;
	    p.next = null;

	} else {

	    for ( int i=0; i<pos; i++ ) // traversing pos nodes
		if ( p == null )
		    throw new IndexOutOfBoundsException(Integer.toString(pos));
		else
		    p = p.next;
	    
	    Node del = p;  // the node to delete

	    p = p.previous; // p designates de previous node

	    Node q = del.next; // q designates the node that follows

	    p.next = q;

	    if ( del == tail )
		tail = p;
	    else
		q.previous = p;

	    del.value = null;
	    del.next = null;
	    del.previous = null;
	}
    }

}