Try this:
<div id="category-list">
<ul class="topnav">
<?php
if (is_single()){
global $post;
$pid = $post->ID;
$post_categories = wp_get_post_categories( $post->ID );
$cats = array();
foreach($post_categories as $c){
$cat = get_category( $c );
$cats[] = $cat->ID;
}
$active_cat_count = true;
$cat_count = 0;
}
$kategorien = get_categories(array(
'child_of' => 0,
'exclude_tree' => 15,
'exclude' => 16,
'orderby' => 'slug',
'order' => 'ASC'));
foreach($kategorien as $kategorie) {
if (isset($active_cat_count) && $active_cat_count){
if (in_array($kategorie->ID,$cats){
$active_cat_count = false;
}else{
$cat_count = $cat_count + 1;
}
}
echo '<li><a href="#" class="category-title">'; echo ($kategorie->name); echo '</a>';
echo '<ul>';
$cat = $kategorie->cat_ID;
$posts = get_posts('category='. $cat .'&posts_per_page=-1');
foreach($posts as $post) { ?>
<li>
<a <?php if ($post->ID == $pid){echo 'class="current"';} ?> href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
<li>
<? }
echo '</ul>';
echo '</li>';
}
?>
</ul>
</div>
<?php
if (is_single()){
?>
<script>
jQuery(document).ready(function(){
$( ".topnav" ).accordion( "option", "active", <?php echo $cat_count; ?> );
});
</script>
<?php
}