"variable">
            <tr>
                <th class="scene_desc" colspan="3">数据</th>
            </tr>
            <tr>
                <td class="symbol"><img src="variable_L.svg"/></td>
                <td>访问顺序</td>
                <td class="code">L</td>
            </tr>
            </tbody>
        </table>
        <br/>
        <table class="items" cellspacing="0">
            <tbody id="formula">
            <tr>
                <th class="scene_desc" colspan="3">二叉树的遍历</th