      "    * Yes\n",
      "* Can we assume we already have a Node class?\n",
      "    * Yes"
     ]
    },
    {
 
</s> remove     "* If the node is None, return False\n",
 </s> add     "* If the node is None, return True\n", </s> remove     "    if not __validate_bst__(node.right, node.data, maximum):\n",
 </s> add     "    if not _validate_bst(node.right, node.data, maximum):\n",
    "    * Yes\n",
    "* Can we assume this fits in memory?\n",
      "    * Yes\n",
      "* Can we assume we already have a Node class?\n",
    "    * Yes\n",
    "* Can we assume this fits in memory?\n",
      "    * Yes"
     ]
    },
    {
keep add 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_challenge.ipynb