flatten.rb

array = ["america", "europe", "asia", "africa", "france", "usa", "spain", "paris", "los angeles"]

definitions = {
  "continent" => ["america", "europe", "asia", "africa"],
  "country" => ["france", "usa", "spain"],
  "city" => ["paris", "los angeles"],
  "planet" => ["mars","earth"]
}

inverse_array = definitions.map {|k,v| v.map { |e| [e, k]}}.flatten(1)
inverse_hash = Hash[inverse_array]

output = array.map { |e| inverse_hash[e] }
puts output.inspect