      "* Is this a binary search tree?\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"
 
</s> remove     "* 5, 3, 8, 2, 4, 1, 7, 6, 9, 10, 11 -> [[5], [3, 8], [2, 4, 7, 9], [1, 6, 10], [11]]"
 </s> add     "* 5, 3, 8, 2, 4, 1, 7, 6, 9, 10, 11 -> [[5], [3, 8], [2, 4, 7, 9], [1, 6, 10], [11]]\n",
    "\n",
    "Note: Each number in the result is actually a node containing the number" </s> remove     "* 5, 3, 8, 2, 4, 1, 7, 6, 9, 10, 11 -> [[5], [3, 8], [2, 4, 7, 9], [1, 6, 10], [11]]"
 </s> add     "* 5, 3, 8, 2, 4, 1, 7, 6, 9, 10, 11 -> [[5], [3, 8], [2, 4, 7, 9], [1, 6, 10], [11]]\n",
    "\n",
    "Note: Each number in the result is actually a node containing the number"
    "* Should each level be a list of nodes?\n",
    "    * Yes\n",
      "* Is this a binary search tree?\n",
      "    * Yes\n",
    "* Should each level be a list of nodes?\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"
keep add keep keep keep keep
 Polish tree level lists challenge and solution (#79)

Update constraints and test cases. 
graphs_trees/tree_level_lists/tree_level_lists_challenge.ipynb