      "## Constraints\n",
      "\n",
      "* Can the tree have duplicates?\n",
      "    * Yes\n",
      "* Can we assume we already have a Node class?\n",
      "    * Yes\n",
      "* Can we assume this fits in memory?\n",
      "    * Yes"
     ]
    },
 
</s> remove     "* If the node is None, return False\n",
 </s> add     "* If the node is None, return True\n",
    "* If this is called on a None input, should we raise an exception?\n",
    "    * Yes\n",
      "## Constraints\n",
      "\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"
     ]
    },
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_challenge.ipynb