# 二叉树的最大深度

<p>给定一个二叉树，找出其最大深度。</p>

<p>二叉树的深度为根节点到最远叶子节点的最长路径上的节点数。</p>

<p><strong>说明:</strong>&nbsp;叶子节点是指没有子节点的节点。</p>

<p><strong>示例：</strong><br>
给定二叉树 <code>[3,9,20,null,null,15,7]</code>，</p>

<pre>    3
   / \
  9  20
    /  \
   15   7</pre>

<p>返回它的最大深度&nbsp;3 。</p>


## template

```python
class TreeNode:
     def __init__(self, x):
         self.val = x
         self.left = None
         self.right = None

class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        if root is None:
            return 0
        maxdepth = max(self.maxDepth(root.left),self.maxDepth(root.right)) + 1
        return maxdepth   

```

## 答案

```python

```

## 选项

### A

```python

```

### B

```python

```

### C

```python

```