# 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 maximum_depth_of_binary_tree(tree_node) if tree_node.nil? 0 else 1 + [maximum_depth_of_binary_tree(tree_node.left), maximum_depth_of_binary_tree(tree_node.right)].max end end