Implementation of preorder, inorder and postorder in a binary tree

tree_traversal.rb

require 'rspec'
require 'forwardable'
 
RSpec.configure do |config|
  config.mock_with :rspec
  config.color_enabled = true
end

class Node
  def initialize(value)
    @left = nil
    @right = nil
    @value = value
  end
  
  def add_left(value)
    @left = Node.new(value)
  end
  
  def add_right(value)
    @right = Node.new(value)
  end
  
  def inorder
    [(@left.inorder if @left), @value, (@right.inorder if @right)].flatten.compact
  end
  
  def preorder
    [@value, (@left.preorder if @left), (@right.preorder if @right)].flatten.compact
  end

  def postorder
    [(@left.postorder if @left), (@right.postorder if @right), @value].flatten.compact
  end
end

class BinaryTree
  extend Forwardable
  
  attr_reader :root
  def_delegators :"@root", :inorder, :preorder, :postorder
  
  def initialize(root_value)
    @root = Node.new(root_value)
  end
end

describe BinaryTree do
  subject { BinaryTree.new(4) }
  
  before do
    node_2 = subject.root.add_left(2)
    node_2.add_left(1)
    node_2.add_right(3)
    
    node_6 = subject.root.add_right(6)
    node_6.add_left(5)
    node_6.add_right(7) 
  end
  
  describe "#inorder" do
    it "return node values" do
      subject.inorder.should eq([1, 2, 3, 4, 5, 6, 7])
    end
  end

  describe "#preorder" do
    it "return node values" do
      subject.preorder.should eq([4, 2, 1, 3, 6, 5, 7])
    end
  end

  describe "#postorder" do
    it "return node values" do
      subject.postorder.should eq([1, 3, 2, 5, 7, 6, 4])
    end
  end
end