# 136.Maximum Depth of Binary Tree

## Description

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.

For example:
Given binary tree `[3,9,20,null,null,15,7]`, return its depth = 3.

## From

[LeetCode](https://leetcode.com/problems/maximum-depth-of-binary-tree)
