# Definition for a binary tree node. # class TreeNode # attr_accessor :val, :left, :right # def initialize(val = 0, left = nil, right = nil) # @val = val # @left = left # @right = right # end # end # @param {TreeNode} root # @return {Integer} def max_depth(root) depth(root, 0) end def depth(node, height) if node.nil? return height else [depth(node.left, height+1), depth(node.right, height+1)].max end end