class Lista
            
             attr_accessor :top, :last
             Node = Struct.new(:value, :prev, :next)
               
             def initialize(top)
                if top === nil
                  @top = nil
                  @last = nil
                else
                   @top = Node.new(top,nil,nil)
                   @last = Node.new(top,nil,nil)
                end
             end
             def insertarMultiple(elementos)
                 elementos.each do |elemento|
                        insertar(elemento)
                 end
             end
             def insertar(elemento)
                if empty
                     @top = Node.new(elemento,nil, nil)
                     @last =  Node.new(elemento,nil, nil)
                else
                     @top[:next] = elemento;
                     nmenu = Node.new(elemento, @top, nil)
                     @top = nmenu
                end
             end
              
             def mostrarUltimo
                if empty
                   puts "Lista vacia. Empty!"
                else
                    aux = @top;
                    while aux[:prev] != nil
        			aux = aux[:prev]
                    end
                    
                    aux[:value]
                    
                end
                
             end
             
             def getPrevTop
                 aux = top[:prev]
                 val = aux[:value]
                 val
             end
             
             def extraer
                if empty
                   puts "Lista vacia. Empty!"
                else
                    extraer = @top[:value]
                    aux_to = @top[:prev]
                    
                    if @top == @last
                      @last = nil;
                    end
                    
                    aux_to[:next] = nil
                    @top = aux_to
                    
                    extraer
                end
                
             end
             
             def empty
               if @top == nil
                  true
               else
                  false
               end
             end
        
        end