怎么实现一级栏目有子栏目时 添加css class属性

        {list action=category module=share pid=0}  
          <li><a href="{$t.url}">{$t.name} <span class="caret"></span></a>
           {if $t.child}
            <ul class="dropdown-menu">
             {list action=category module=share pid=$t.id return=t2}
              <li><a href="{$t2.url}">{$t2.name} <span class="caret"></span></a>
              {if $t2.child}
                <ul class="dropdown-menu">
                  {list action=category module=share pid=$t2.id return=t3}
                  <li><a href="{$t3.url}">{$t3.name}</a></li>
                  {/list}
                </ul>
                {/if}
              </li>
               {/list}
            </ul>
            {/if}
          </li>
{/list}