     "metadata": {},
     "source": [
      "## Constraints\n",
      "\n",
      "* None"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
 
</s>
    "* 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_solution.ipynb