jQuery在单击时添加类，在单击另一个时删除

I have the markup like this

我有这样的标记

<div class="personal-menu-content">
    <ul>
         <li><a data-menu-item="lessons" class="personal-menu-item lessons" href="/go.html?url=#">Lessons</a></li>
         <li><a data-menu-item="profile" class="personal-menu-item profile" href="/go.html?url=#">Edit Profile</a></li>
         <li><a data-menu-item="library" class="personal-menu-item library" href="/go.html?url=#">Your Library</a></li>
    </ul>
</div>

<div class="contents">
    <div id="lessons">
        <h2>Lessons text here</h2>
        <p>Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s</p>
    </div>
    <div id="profile">
        <h2>profile text here</h2>
        <p>It is a long established fact that a reader will be distracted by the readable content of a page when looking at its layout. The point of using Lorem Ipsum is that it has a more-or-less normal distribution of letters, as opposed to using 'Content here, content here', making it look like readable English.</p>
    </div>
    <div id="library">
        <h2>library text here</h2>
        <p>Contrary to popular belief, Lorem Ipsum is not simply random text. It has roots in a piece of classical Latin literature from 45 BC, making it over 2000 years old. Richard McClintock, a Latin professor at Hampden-Sydney College in Virginia, looked up one of the more obscure Latin words,</p>
    </div>
</div>
<div cl