Polish check tree balance challenge and solution (#80)

Update constraints, test cases, tests, and code.

graphs_trees/check_balance/check_balance_challenge.ipynb

@@ -36,7 +36,11 @@
     "\n",
     "* Is a balanced tree one where the heights of two sub trees of any node doesn't differ by more than 1?\n",
     "    * Yes\n",
+    "* If this is called on a None input, should we return False?\n",
+    "    * Yes\n",
     "* Can we assume we already have a Node class with an insert method?\n",
+    "    * Yes\n",
+    "* Can we assume this fits memory?\n",
     "    * Yes"
    ]
   },
@@ -46,6 +50,8 @@
    "source": [
     "## Test Cases\n",
     "\n",
+    "* None -> No\n",
+    "* 1 -> Yes\n",
     "* 5, 3, 8, 1, 4 -> Yes\n",
     "* 5, 3, 8, 9, 10 -> No"
    ]
@@ -120,7 +126,11 @@
     "class TestCheckBalance(object):\n",
     "\n",
     "    def test_check_balance(self):\n",
+    "        assert_equal(check_balance(None), False)\n",
+    "\n",
     "        node = Node(5)\n",
+    "        assert_equal(check_balance(node), True)\n",
+    "\n",
     "        insert(node, 3)\n",
     "        insert(node, 8)\n",
     "        insert(node, 1)\n",
@@ -134,6 +144,15 @@
     "        insert(node, 10)\n",
     "        assert_equal(check_balance(node), False)\n",
     "\n",
+    "        node = Node(3)\n",
+    "        insert(node, 2)\n",
+    "        insert(node, 1)\n",
+    "        insert(node, 5)\n",
+    "        insert(node, 4)\n",
+    "        insert(node, 6)\n",
+    "        insert(node, 7)\n",
+    "        assert_equal(check_balance(node), False)\n",
+    "\n",
     "        print('Success: test_check_balance')\n",
     "\n",
     "\n",
@@ -158,21 +177,21 @@
  ],
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-   "display_name": "Python 2",
+   "display_name": "Python 3",
    "language": "python",
-   "name": "python2"
+   "name": "python3"
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   "language_info": {
    "codemirror_mode": {
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-    "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"
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  "nbformat": 4,