class Node
    attr_reader   :data
    attr_accessor :next
    
    def initialize(data)
        @data = data   
    end
    
    def to_s
        output = (@next != nil) ? "#{@next}, " : ""
        output += "#{@data}"
        output
    end
end
class Stack
    attr_reader :top

    def initialize
        puts self.to_s
    end

    def push(data)
        node = Node.new(data)
        node.next = @top 
        @top = node       
        puts "push #{data}"
        puts self.to_s
        return data
    end

    def pop
        if @top == nil
            return nil
        end
        data = @top.data
        @top = @top.next  
        puts "pop #{data}"
        puts self.to_s
        return data
    end

    def to_s
        "Stack: [#{@top}]"
    end
end

stack = Stack.new
stack.pop
(1..3).each { |i| stack.push(i) }
5.times { stack.pop }