/**
 * A single linked list.
 * 
 * @author  Josef Nilsen
 * @version v1.0B
 */
public class LinkedList<T> implements Stack<T>
{ 
    private ListElement<T> first;   // First element in list.
    private int size;               // Number of elements in list.
    
    /**
     * A list element.
     */
    private static class ListElement<T>
    {
        public T data;
        public ListElement<T> next;
        
        public ListElement(T data) {
            this.data = data;
            this.next = null;
        }
    }
    
    /**
     * Creates an empty list.
     */
    public LinkedList() {
    }

    /**
     * Inserts the given element at the beginning of this list.
     */
    public void push(T element) {
        if(size==0) {
            first = new ListElement<T>(element);
        } else {
            ListElement<T> tmp = first;
            first = new ListElement<T>(element);
            first.next = tmp;
        }
        size++;
    }

    /**
     * Returns the first element of this list.
     * Returns <code>null</code> if the list is empty.
     */
    public T pop() {
        if(size==0) {
            return null;
        } else {
            ListElement<T> tmp = first;
            first = tmp.next;
            return tmp.data;
        }
    }
    
    public T top() {
    	if(size==0) {
    		return null;
    	} else {
    		return first.data;
    	}
    }

    /**
     * Returns the number of elements in this list.
     */
    public int size() {
        return size;
    }

    /**
     * Returns <code>true</code> if this list contains no elements.
     */
    public boolean isEmpty() {
        return size == 0;
    }
 }