（难度Easy） Problem 104. Maximum Depth of Binary Tree（二叉树最大深度）

Maximum Depth of Binary Tree

Easy

## 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.

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.