     "metadata": {},
     "source": [
      "## Test Cases\n",
      "\n",
      "<pre>\n",
      "Valid:\n",
      "      5\n",
      "    /   \\\n",
      "   5     8\n",
      "  / \\   /\n",
 
</s> remove     "    return __validate_bst__(node, None, None)\n",
 </s> add     "    if node is None:\n",
    "        raise Exception('No root node')\n",
    "    return _validate_bst(node, None, None)\n",
    "None -> exception\n",
    "\n",
     "metadata": {},
     "source": [
      "## Test Cases\n",
      "\n",
    "None -> exception\n",
    "\n",
      "<pre>\n",
      "Valid:\n",
      "      5\n",
      "    /   \\\n",
      "   5     8\n",
      "  / \\   /\n",
keep keep keep add keep keep keep keep keep keep
 Polish bst validate challenge and solution (#83)

Update constraints, test cases, tests, algorithm discussion, and code. 
graphs_trees/bst_validate/bst_validate_solution.ipynb