Fix #13, PEP8-ify notebooks.

stacks_queues/stack/stack_challenge.ipynb

@@ -35,7 +35,6 @@
    "source": [
     "## Constraints\n",
     "\n",
-
     "* None"
    ]
   },
@@ -92,13 +91,14 @@
    "outputs": [],
    "source": [
     "class Node(object):\n",
-    "    \n",
+    "\n",
     "    def __init__(self, data):\n",
     "        # TODO: Implement me\n",
     "        pass\n",
     "\n",
+    "\n",
     "class Stack(object):\n",
-    "    \n",
+    "\n",
     "    def __init__(self, top=None):\n",
     "        # TODO: Implement me\n",
     "        pass\n",
@@ -144,7 +144,7 @@
     "\n",
     "\n",
     "class TestStack(object):\n",
-    "    \n",
+    "\n",
     "    # TODO: It would be better if we had unit tests for each\n",
     "    # method in addition to the following end-to-end test\n",
     "    def test_end_to_end(self):\n",
@@ -172,13 +172,15 @@
     "        assert_equal(stack.pop(), 1)\n",
     "        assert_equal(stack.peek(), None)\n",
     "        assert_equal(stack.is_empty(), True)\n",
-    "        \n",
+    "\n",
     "        print('Success: test_end_to_end')\n",
     "\n",
+    "\n",
     "def main():\n",
     "    test = TestStack()\n",
     "    test.test_end_to_end()\n",
-    "    \n",
+    "\n",
+    "\n",
     "if __name__ == '__main__':\n",
     "    main()"
    ]