fibo_dictionary.rb

class FiboUsingDictionary                                                            
  include Singleton                                                                  
                                                                                     
  def initialize                                                                     
    @fib = { 0 => 1, 1 => 1 }                                                        
  end                                                                                
                                                                                     
  def [](n)                                                                          
    return @fib[n] if @fib.include? n                                                
    @fib[n] = self[n -1] + self[n - 2]                                               
  end                                                                                
end