public interface BinTree<E> {
    public boolean isEmpty();       // is tree empty?
    public int size();          // number of elements
    public int height();            // maximal depth
    public java.util.List<E> preOrder();      // pre-order traversal
    public java.util.List<E> inOrder();       // in-order traversal
    public java.util.List<E> postOrder(); // post-order traversal
}