<p>请完成一个函数，输入一个二叉树，该函数输出它的镜像。</p>

<p>例如输入：</p>

<p><code>&nbsp; &nbsp; &nbsp;4<br>
&nbsp; &nbsp;/ &nbsp; \<br>
&nbsp; 2 &nbsp; &nbsp; 7<br>
&nbsp;/ \ &nbsp; / \<br>
1 &nbsp; 3 6 &nbsp; 9</code><br>
镜像输出：</p>

<p><code>&nbsp; &nbsp; &nbsp;4<br>
&nbsp; &nbsp;/ &nbsp; \<br>
&nbsp; 7 &nbsp; &nbsp; 2<br>
&nbsp;/ \ &nbsp; / \<br>
9 &nbsp; 6 3&nbsp; &nbsp;1</code></p>

<p>&nbsp;</p>

<p><strong>示例 1：</strong></p>

<pre><strong>输入：</strong>root = [4,2,7,1,3,6,9]
<strong>输出：</strong>[4,7,2,9,6,3,1]
</pre>

<p>&nbsp;</p>

<p><strong>限制：</strong></p>

<p><code>0 &lt;= 节点个数 &lt;= 1000</code></p>

<p>注意：本题与主站 226 题相同：<a href="https://leetcode-cn.com/problems/invert-binary-tree/">https://leetcode-cn.com/problems/invert-binary-tree/</a></p>