Given a binary tree, find its maximum depth.求二叉树的最大深度
The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.
用递归：
Given a binary tree, find its maximum depth.求二叉树的最大深度
The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.
用递归：