node.rb

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

def traverse(node, k, depth=0)
  return if k < 0
  return if depth > k
  if depth == k
    puts node.inspect
  elsif depth < k
    if node.left
      traverse(node.left, k, depth+1)
    end
    if node.right
      traverse(node.right, k, depth+1)
    end
  end
end

def testrunner
  n = Node.new(0, 'zero')
  ##
  n.left = Node.new(1, 'l-1')
  n.right = Node.new(1, 'r-1')
  ##
  n.left.left = Node.new(2, 'l-l')
  n.left.right = Node.new(2, 'l-r')

  n.right.left = Node.new(2, 'r-l')
  n.right.right = Node.new(2, 'r-r')
  ##
  n.left.left.left = Node.new(3, 'l-l-l')
  n.left.left.right = Node.new(3, 'l-l-r')

  n.left.right.left = Node.new(3, 'l-r-l')
  n.left.right.right = Node.new(3, 'l-r-l')

  n.right.left.left = Node.new(3, 'r-l-l')
  n.right.left.right = Node.new(3, 'r-l-r')
  n.right.right.left = Node.new(3, 'r-r-l')
  n.right.right.right = Node.new(3, 'r-r-r')

  ##


  traverse(n, 3, 0)
  traverse(n, 2, 0)
  traverse(n, 1, 0)
  traverse(n, 0, 0)

end
testrunner

Ruby Code to Print All Nodes in a Tree at Depth [k]