I accidentally got the answer:
<?php
global $post;
$the_post_parent = $post->post_parent;
$the_post_ID = $post->ID;
$cat_posts = get_posts('post_type=bbp_forum&posts_per_page=-1');
print_r($post_parent);
?>
<?php foreach ( $cat_posts as $post ) : ?>
<li <?php if ( $post->ID == $the_post_ID || $post->ID == $the_post_parent ) echo 'class="current"'; ?>>
<a href="https://wordpress.stackexchange.com/questions/13966/<?php the_permalink(); ?>" title="<?php printf( esc_attr__( 'Permalink to %s', 'twentyten' ), the_title_attribute( 'echo=0' ) ); ?>" rel="bookmark"><?php the_title(); ?></a>
</li>
<?php endforeach; ?>
I would appreciate if someone explain it to me.