Polish stack min challenge and solution (#72)

Update constraints, algorithm discussion, code, and tests.

stacks_queues/stack_min/stack_min_challenge.ipynb

@@ -36,9 +36,13 @@
     "\n",
     "* Can we assume this is a stack of ints?\n",
     "    * Yes\n",
-    "* If we call this function on an empty stack, can we return maxsize?\n",
+    "* Can we assume the input values for push are valid?\n",
+    "    * Yes\n",
+    "* If we call this function on an empty stack, can we return sys.maxsize?\n",
     "    * Yes\n",
     "* Can we assume we already have a stack class that can be used for this problem?\n",
+    "    * Yes\n",
+    "* Can we assume this fits memory?\n",
     "    * Yes"
    ]
   },
@@ -91,7 +95,7 @@
     "import sys\n",
     "\n",
     "\n",
-    "class MyStack(Stack):\n",
+    "class StackMin(Stack):\n",
     "\n",
     "    def __init__(self, top=None):\n",
     "        # TODO: Implement me\n",
@@ -137,7 +141,7 @@
     "\n",
     "    def test_stack_min(self):\n",
     "        print('Test: Push on empty stack, non-empty stack')\n",
-    "        stack = MyStack()\n",
+    "        stack = StackMin()\n",
     "        stack.push(5)\n",
     "        assert_equal(stack.peek(), 5)\n",
     "        assert_equal(stack.min(), 5)\n",
@@ -188,21 +192,21 @@
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Python 2",
+   "display_name": "Python 3",
    "language": "python",
-   "name": "python2"
+   "name": "python3"
   },
   "language_info": {
    "codemirror_mode": {
     "name": "ipython",
-    "version": 2
+    "version": 3
    },
    "file_extension": ".py",
    "mimetype": "text/x-python",
    "name": "python",
    "nbconvert_exporter": "python",
-   "pygments_lexer": "ipython2",
-   "version": "2.7.10"
+   "pygments_lexer": "ipython3",
+   "version": "3.5.0"
   }
  },
  "nbformat": 4,