      "        * Add node's value to the hash map\n",
      "\n",
      "Complexity:\n",
      "* Time: O(n)\n",
      "* Space: O(m) where m is the number of values in the hash map\n",
      "\n",
      "Note:\n",
      "* Deletion requires two pointers, one to the previous node and one to the current node"
     ]
    },
 
</s> remove     "* Can you insert NULL values in the list?\n",
 </s> add     "* Can you insert None values in the list?\n", </s> remove     "* Can you insert NULL values in the list?\n",
 </s> add     "* Can you insert None values in the list?\n", </s> remove     "* Space: In-place\n",
 </s> add     "* Space: O(1)\n",
    "* Space: O(n)\n",
      "        * Add node's value to the hash map\n",
      "\n",
      "Complexity:\n",
      "* Time: O(n)\n",
    "* Space: O(n)\n",
      "\n",
      "Note:\n",
      "* Deletion requires two pointers, one to the previous node and one to the current node"
     ]
    },
keep keep keep keep replace keep keep keep keep keep
 Tweaked remove duplicates challenge algorithm discussion. 
linked_lists/remove_duplicates/remove_duplicates_solution.ipynb