<h2><a href="https://leetcode.com/problems/binary-tree-level-order-traversal/">102. Binary Tree Level Order Traversal</a></h2><h3>Medium</h3><hr><div><p>Given the <code>root</code> of a binary tree, return <em>the level order traversal of its nodes' values</em>. (i.e., from left to right, level by level).</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/02/19/tree1.jpg" style="width: 277px; height: 302px;">
<pre><strong>Input:</strong> root = [3,9,20,null,null,15,7]
<strong>Output:</strong> [[3],[9,20],[15,7]]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre><strong>Input:</strong> root = [1]
<strong>Output:</strong> [[1]]
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre><strong>Input:</strong> root = []
<strong>Output:</strong> []
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the tree is in the range <code>[0, 2000]</code>.</li>
	<li><code>-1000 &lt;= Node.val &lt;= 1000</code></li>
</ul>
</div>