Commit c63531f

@@ -0,0 +1,59 @@
+class LinkedListStack
+  def initialize
+    @head = NullNode.new
+  end
+
+  def push(item)
+    @head.push(item)
+  end
+
+  def pop
+    @head.pop
+  end
+end
+
+class Node
+  attr_reader :data
+
+  def initialize(data)
+    @data = data
+  end
+
+  def push(item)
+    if @next 
+      @next.push(item)
+    end
+
+    @next = Node.new(item)
+  end
+
+  def pop
+    if @next
+      if @next.is_tail?
+        result = @next.data
+        @next = nil
+        result
+      else
+        @next.pop
+      end
+    end
+  end
+
+  def is_tail?
+    @next == nil
+  end
+end
+
+class NullNode
+  def push(item)
+    if @next
+      @next.push(item)
+    else
+      @next = Node.new(item)
+    end
+  end
+
+  def pop
+    return @next.pop if @next
+  end
+end

@@ -0,0 +1,31 @@
+require "spec_helper"
+
+describe LinkedListStack do
+  let(:sut) { LinkedListStack.new }
+
+  context "when no items are pushed on to the stack" do
+    let(:result) { sut.pop }
+
+    it "should pop nil if there is nothing on the stack" do
+      result.should be_nil
+    end
+  end
+
+  context "when a single item is pushed on to the stack" do
+    it "should pop the last item pushed on to the stack" do
+      sut.push(1)
+      sut.push(2)
+      result = sut.pop
+      result.should == 2
+    end
+
+    it "should pop off each item in reverse order of how they were put on" do
+      (1..10).each do |n|
+        sut.push(n)
+      end
+      (10..1).each do |n|
+        sut.pop.should == n
+      end
+    end
+  end
+end

@@ -3,3 +3,5 @@ require_relative '../lib/sorting/bubble_sort'
 require_relative '../lib/sorting/insertion_sort'
 require_relative '../lib/sorting/quick_sort'
 require_relative '../lib/sorting/merge_sort'
+
+require_relative '../lib/data_structures/linked_list_stack'