# 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...