<p>给你一棵二叉树的根节点 <code>root</code> ，翻转这棵二叉树，并返回其根节点。</p>

<p>&nbsp;</p>

<p><strong>示例 1：</strong></p>

<p><img alt="" src="https://assets.leetcode.com/uploads/2021/03/14/invert1-tree.jpg" style="height: 165px; width: 500px;" /></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><strong>示例 2：</strong></p>

<p><img alt="" src="https://assets.leetcode.com/uploads/2021/03/14/invert2-tree.jpg" style="width: 500px; height: 120px;" /></p>

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

<p><strong>示例 3：</strong></p>

<pre>
<strong>输入：</strong>root = []
<strong>输出：</strong>[]
</pre>

<p>&nbsp;</p>

<p><strong>提示：</strong></p>

<ul>
	<li>树中节点数目范围在 <code>[0, 100]</code> 内</li>
	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
</ul>