leetcode 第二题

Maximum Depth of Binary Tree

题目地址：https://leetcode.com/problems/maximum-depth-of-binary-tree/

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.
题目描述：找出二叉树中最深节点的深度

利用递归即可。

Ruby 版：

# Definition for a binary tree node.
# class TreeNode
#     attr_accessor :val, :left, :right
#     def initialize(val)
#         @val = val
#         @left, @right = nil, nil
#     end
# end

# @param {TreeNode} root
# @return {Integer}
def max_depth(root)
    root.nil? ? 0 : [max_depth(root.left) + 1, max_depth(root.right) + 1].max
end