div class="dropup pull-right">
    <button class="btn btn-default dropdown-toggle" data-toggle="dropup" id="dropmenus" aria-haspopup="true"
            aria-expanded="true">下拉菜单</button>
    <ul class="dropdown-menu pull-right" role="menu" aria-labelledby="dropmenus" id="menus">
        <li class=" dropdown-header" role="presentation">第一部分头</li>
        <li role="presentation"><a href="#">苹果</a></li>
        <li role="presentation" class="active"><a href="#">香蕉</a></li>
        <li role="presentation"><a href="#">西瓜</a></li>
        <li class="dropdown-header" role="presentation">第二部分头</li>
        <li role="presentation"><a href="#">西红柿</a></li>
        <li role="presentation"><a href="#">猕猴桃</a></li>
        <li role="presentation"><a href="#">火龙果</a></li>
    </ul>
</div>