      "* Is this a singly or doubly linked list?\n",
      "    * Singly\n",
      "* Can you insert None values in the list?\n",
      "    * No\n",
      "* Can you use additional data structures?\n",
      "    * Implement both solutions\n",
      "* Can we assume we already have a linked list class that can be used for this problem?\n",
      "    * Yes"
     ]
    },
    {
 
</s> remove     "* Can you use additional data structures?\n",
    "    * Implement both solutions\n",
 </s> add  </s> remove     "        * If the node's value is in the hash map\n",
    "            * Delete the node\n",
    "        * Else\n",
    "            * Add node's value to the hash map\n",
 </s> add     "        * Delete nodes that match current node\n",
      "* Is this a singly or doubly linked list?\n",
      "    * Singly\n",
      "* Can you insert None values in the list?\n",
      "    * No\n",


      "* Can we assume we already have a linked list class that can be used for this problem?\n",
      "    * Yes"
     ]
    },
    {
keep keep keep keep replace replace keep keep keep keep keep
 Polish remove duplicates challenge and solution

Update constraints and algorithm discussion. 
linked_lists/remove_duplicates/remove_duplicates_challenge.ipynb