# 在每个树行中找最大值

<p>给定一棵二叉树的根节点 <code>root</code> ，请找出该二叉树中每一层的最大值。</p>

<p> </p>

<p><strong>示例1：</strong></p>

<pre>
<strong>输入: </strong>root = [1,3,2,5,3,null,9]
<strong>输出: </strong>[1,3,9]
<strong>解释:</strong>
          1
         / \
        3   2
       / \   \  
      5   3   9 
</pre>

<p><strong>示例2：</strong></p>

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

<p><strong>示例3：</strong></p>

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

<p><strong>示例4：</strong></p>

<pre>
<strong>输入: </strong>root = [1,null,2]
<strong>输出: </strong>[1,2]
<strong>解释:</strong>      
           1 
            \
             2     
</pre>

<p><strong>示例5：</strong></p>

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

<p> </p>

<p><strong>提示：</strong></p>

<ul>
	<li>二叉树的节点个数的范围是 <code>[0,10<sup>4</sup>]</code></li>
	<li><meta charset="UTF-8" /><code>-2<sup>31</sup> <= Node.val <= 2<sup>31</sup> - 1</code></li>
</ul>

<p>以下错误的选项是？</p>

## aop

### before

```cpp

```
### after

```cpp

```

## 答案

```cpp

```

## 选项


### A

```cpp

```

### B

```cpp

```

### C

```cpp

```
