     "metadata": {},
     "source": [
      "## Constraints\n",
      "\n",
      "* None"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
 
</s> remove     "* None"
 </s> add     "* If we pop on an empty stack, do we return None?\n",
    "    * Yes\n",
    "* Can we assume this fits memory?\n",
    "    * Yes" </s> remove    "display_name": "Python 2",
 </s> add    "display_name": "Python 3", </s> remove    "name": "python2"
 </s> add    "name": "python3" </s> remove     "version": 2
 </s> add     "version": 3 </s> remove     "        node = Node(data, self.top)\n",
    "        self.top = node\n",
 </s> add     "        self.top = Node(data, self.top)\n",
    "* If we pop on an empty stack, do we return None?\n",
    "    * Yes\n",
    "* Can we assume this fits memory?\n",
    "    * Yes"
     "metadata": {},
     "source": [
      "## Constraints\n",
      "\n",
    "* If we pop on an empty stack, do we return None?\n",
    "    * Yes\n",
    "* Can we assume this fits memory?\n",
    "    * Yes"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
keep keep keep keep replace keep keep keep keep keep
 Polish stack challenge and solution (#69)

Update constraints and code. 
stacks_queues/stack/stack_challenge.ipynb