Polish stack challenge and solution (#69)

Update constraints and code.

stacks_queues/stack/stack_solution.ipynb

@@ -34,7 +34,10 @@
    "source": [
     "## Constraints\n",
     "\n",
-    "* None"
+    "* If we pop on an empty stack, do we return None?\n",
+    "    * Yes\n",
+    "* Can we assume this fits memory?\n",
+    "    * Yes"
    ]
   },
   {
@@ -148,8 +151,7 @@
     "        self.top = top\n",
     "\n",
     "    def push(self, data):\n",
-    "        node = Node(data, self.top)\n",
-    "        self.top = node\n",
+    "        self.top = Node(data, self.top)\n",
     "\n",
     "    def pop(self):\n",
     "        if self.top is None:\n",
@@ -319,7 +321,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.4.3"
+   "version": "3.5.0"
   }
  },
  "nbformat": 4,