stack.rb

class Node
    attr_accessor :value, :next_node
    
    def initialize(value, next_node = nil)
        @value = value
        @next_node = next_node
    end
end
class LinkedList
    def initialize
        @head = nil
    end
    
    def add_at(index, item)
      if index == 0
        @head = Node.new(item, @head)
      else
        nodeprev = get_node(index-1)
        node = Node.new(item, nodeprev.next_node)
        nodeprev.next_node = node
      end
    end
    
    def remove(index)
      if @head.nil?
        puts "error"
      elsif index == 0
        node = @head
        new_head = node.next_node
        @head = new_head
      else
        node = get_node(index - 1)
        node.next_node = get_node(index + 1)
      end
    end
  
    def add(number)
      if @head.nil?
        @head = Node.new(number)
      else
        node = @head
        while node.next_node != nil
          node = node.next_node
        end
        node.next_node = Node.new(number)
      end
    end
  
    def get(index)
      node = @head
      index.times do
        node = node.next_node
      end
      return node.value
    end
    
    private
    
    def get_node(index)
      node = @head
      index.times do
        node = node.next_node
      end
      return node
    end
  end

class Stack
  def initialize
    @head = nil
  end
  def push(number)
    if @head.nil?
      @head = Node.new(number, @head)
    else
      new_head = Node.new(number, @head)
      @head = new_head
    end
  end
  
  def pop
    if @head.nil?
      puts "error, is empty"
    else
      value = @head.value
      @head = @head.next_node
    end
    value
  end
end

stack = Stack.new
stack.push(3)
stack.push(5)
puts stack.pop
# => 5

stack.push(2)
stack.push(7)
puts stack.pop
# => 7

puts stack.pop
# => 2

puts stack.pop
# => 3