Maximum Depth of Binary Tree

https://leetcode.com/problems/maximum-depth-of-binary-tree/ Easy

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.