Pivot child to parent relationship

hierarchy.rb

@child_and_parent = {
  :b=>:a, 
  :c=>:a, 
  :x=>:a,
  :d=>:b,
  :e=>:b,
  :f=>:c,
  :g=>:c,
  :h=>:g,
  :i=>:h
}

# 'Inject' is Ruby's REDUCE method
@parent_and_children=@child_and_parent.keys.inject({}) do |new_hash,key|
  new_hash[@child_and_parent[key]] ||= []
  new_hash[@child_and_parent[key]] << key
  new_hash
  # Using Ruby 1.9 you can collapse the 3 lines above to one line:
  # new_hash.tap do |this_hash| (this_hash[@child_and_parent[key]] ||= []) << key; end  
end

output.txt

irb(main):021:0> load 'hierarchy.rb'
=> true
irb(main):022:0> @parent_and_children
=> {:a=>[:b, :c, :x], :b=>[:d, :e], :c=>[:f, :g], :g=>[:h], :h=>[:i]}