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...