public class BST<Keys extends Comparable<Key>, Value>
{

  Iterable<Key> keys()
  {
    Queue<Key> keylst = new Queue<Key>();
    addKeys(root, lst);
    return lst;
  }

  /**
   * adds the keys in subtree t in order to the end of lst
   * @param t the subtree
   * @param lst list of keys smaller than the keys in t.
   */

  private void addKeys(Node t, Queue<Key> lst)
  {
    /**
     * (Like printKeys, but add to lst instead of printing:
     *
     * if t is empty  do nothing (just return)
     * add keys from the left subtree
     * add t.key
     * add keys from the right subtree
     */
  }

}