gistfile1.rb

$graph = {a: [:b, :c], b: [:d, :e], c: [:f, :g]}

def dfs_nr(node)
  queue = [node]
  seen = {}
  while not queue.empty?
    node = queue.pop
    puts node
    seen[node] = true
    if $graph[node]
      $graph[node].each {|child| queue.push child unless seen[child] }
    end
  end
end

dfs_nr :a

quejue should be queue?

Yes. Rectified.