Post-order traversal of a binary tree

post_order_traversal.rb

class Node
  attr_accessor :value, :left, :right
  def initialize(value)
    @value = value
  end
end

def post_order_traversal(node)
  post_order_traversal node.left if node.left
  post_order_traversal node.right if node.right
  puts node.value
end

a = Node.new(1)
a.left = Node.new(2)
a.left.left = Node.new(3)
a.left.right = Node.new(4)
a.right = Node.new(5)

post_order_traversal(a) # 3, 4, 2, 5, 1