## Problem

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

```
3
/ \
9 20
/ \
15 7
```

return its depth = 3.

## Thought Process

Can solve this question recursively.

- find the max d for the left tree recursively
- find the max d for the right tree recursively
- return the larger d

## White Board

Below is the white board:

## Code

```
# Definition for a binary tree node.
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
left_d = self.maxDepth(root.left)
right_d = self.maxDepth(root.right)
return max(left_d, right_d) + 1
```

```
```

## Complexity

- Time complexity is O(n)

## Optimization

Maybe have an iterative solution…

```
```

```
```