Fibonaci using array

fibo_array.rb

class FiboUsingArray                                                                 
  include Singleton                                                                  
                                                                                     
  def initialize                                                                     
    @fib = [1, 1]                                                                    
  end                                                                                
                                                                                     
  def [](n)                                                                          
    if n < @fib.size then                                                            
      @fib[n]                                                                        
    else                                                                             
      (@fib.size..n).to_a.each do |i|                                                
        @fib << @fib[-1] + @fib[-2]                                                  
      end                                                                            
      @fib[-1]                                                                       
    end                                                                              
  end                                                                                
                                                                                     
end