Polish bst successor challenge and solution (#82)

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

graphs_trees/bst_successor/bst_successor_challenge.ipynb

@@ -36,7 +36,11 @@
     "\n",
     "* If there is no successor, do we return None?\n",
     "    * Yes\n",
+    "* If the input is None, should we throw an exception?\n",
+    "    * Yes\n",
     "* Can we assume we already have a Node class that keeps track of parents?\n",
+    "    * Yes\n",
+    "* Can we assume this fits memory?\n",
     "    * Yes"
    ]
   },
@@ -57,10 +61,11 @@
     "               /\n",
     "              9\n",
     "\n",
-    "In: 4  Out: 5\n",
-    "In: 5  Out: 6\n",
-    "In: 8  Out: 9\n",
-    "In: 15 Out: None\n",
+    "In: None  Out: Exception\n",
+    "In: 4     Out: 5\n",
+    "In: 5     Out: 6\n",
+    "In: 8     Out: 9\n",
+    "In: 15    Out: None\n",
     "</pre>"
    ]
   },
@@ -129,10 +134,15 @@
    "source": [
     "# %load test_bst_successor.py\n",
     "from nose.tools import assert_equal\n",
+    "from nose.tools import raises\n",
     "\n",
     "\n",
     "class TestBstSuccessor(object):\n",
     "\n",
+    "    @raises(Exception)\n",
+    "    def test_bst_successor_empty(self):\n",
+    "        bst_successor(None)\n",
+    "\n",
     "    def test_bst_successor(self):\n",
     "        nodes = {}\n",
     "        node = Node(5)\n",
@@ -160,6 +170,7 @@
     "def main():\n",
     "    test = TestBstSuccessor()\n",
     "    test.test_bst_successor()\n",
+    "    test.test_bst_successor_empty()\n",
     "\n",
     "\n",
     "if __name__ == '__main__':\n",
@@ -192,7 +203,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.4.3"
+   "version": "3.5.0"
   }
  },
  "nbformat": 4,