class Node
  include Enumerable

  attr_accessor :data, :left, :right
  def initialize(data)
    @data = data
  end

  def each(&block) # turns block into a lambda(proc) object so it can get passed into the method
    left.each(&block) if left # turns the lambda back into a block so it can be used by each
    block.call(self)
    right.each(&block) if right
  end
end

root = Node.new(7)
root.left = Node.new(3) # Nodes to the left of root should be smaller
root.right = Node.new(12) # Nodes to the right of the root should be larger
root.left.left = Node.new(2)
root.left.right = Node.new(6)