/**
     * Destructor for the tree
     */
    ~BinarySearchTree( )
    {
        makeEmpty( );
    }
    /**
     * Internal method to make subtree empty.
     */
    void makeEmpty( BinaryNode * & t )
    {
        if( t != NULL )
        {
            makeEmpty( t->left );
            makeEmpty( t->right );
            delete t;
        }
        t = NULL;
    }