btree_order.rb

=begin
  
      a
     / \
    b   c
   /   / \
  d   e   f

  tree = ['a', ['b', 'd' ], ['c', 'e', 'f']]

  print_tree(tree)
  =>
    a
    b c
    d e f 

  Queue tree root and line break, shift first element, check for 
  left and/or right branches and, if found, then queue up 
  individual branches

=end

def print_tree(tree)
  [tree, "\n"].tap do |q|
    while q.any?
      n = q.shift
      unless n == "\n"
        print "%s " % n[0]
        2.times do |i|
          branch = n.fetch(i+1, nil)
          q << branch unless branch.nil?
        end if n.class == Array
      else
        puts n
        q << n unless q.empty?        
      end   
    end
  end
end