$categories = get_categories(['parent'=>0]);
            foreach($categories as $category):
                $childCategories = get_categories(['child_of' => $category->cat_ID]);
                if(!count($childCategories)){
                    $link = get_category_link($category->term_id);
                    $title = $category->name;
                    printf('<li><a href="%s">%s</a></li>',$link,$title);
                }else{
                    echo '<li>';
                    printf('<a href="#" class="dropdown-toggle" data-toggle="dropdown"
                    role="button" aria-haspopup="true" aria-expanded="false">
                    %s <span class="caret"></span></a>',$category->name);
                    echo '<ul class="dropdown-menu">';
                        foreach($childCategories as $child){
                            $link = get_category_link($child->term_id);
                            $title = $child->name;
                            printf('<li><a href="%s">%s</a></li>',$link,$title);
                        }
                    echo '</li></ul>';
                }
            endforeach;