Polish bst validate challenge and solution (#83)

Update constraints, test cases, tests, algorithm discussion, and code.

graphs_trees/bst_validate/bst_validate_challenge.ipynb

@@ -36,7 +36,11 @@
     "\n",
     "* Can the tree have duplicates?\n",
     "    * Yes\n",
+    "* If this is called on a None input, should we raise an exception?\n",
+    "    * Yes\n",
     "* Can we assume we already have a Node class?\n",
+    "    * Yes\n",
+    "* Can we assume this fits in memory?\n",
     "    * Yes"
    ]
   },
@@ -128,10 +132,15 @@
    "source": [
     "# %load test_bst_validate.py\n",
     "from nose.tools import assert_equal\n",
+    "from nose.tools import raises\n",
     "\n",
     "\n",
     "class TestBstValidate(object):\n",
     "\n",
+    "    @raises(Exception)\n",
+    "    def test_bst_validate_empty(self):\n",
+    "        validate_bst(None)\n",
+    "\n",
     "    def test_bst_validate(self):\n",
     "        node = Node(5)\n",
     "        insert(node, 8)\n",
@@ -149,11 +158,13 @@
     "        root.right = right\n",
     "        root.left.right = invalid\n",
     "        assert_equal(validate_bst(root), False)\n",
+    "\n",
     "        print('Success: test_bst_validate')\n",
     "\n",
     "\n",
     "def main():\n",
     "    test = TestBstValidate()\n",
+    "    test.test_bst_validate_empty()\n",
     "    test.test_bst_validate()\n",
     "\n",
     "\n",
@@ -173,21 +184,21 @@
  ],
  "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,