      "\n",
      "* Is the graph directed?\n",
      "    * Yes\n",
      "* Can we assume we already have Graph and Node classes?\n",
      "    * Yes"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
 
</s> remove     "        if node is None:\n",
    "            continue\n",
 </s> add  </s> remove    "display_name": "Python 2",
 </s> add    "display_name": "Python 3", </s> remove    "display_name": "Python 2",
 </s> add    "display_name": "Python 3", </s> remove    "name": "python2"
 </s> add    "name": "python3"
    "    * Yes\n",
    "* Can we assume the inputs are valid?\n",
    "    * Yes\n",
    "* Can we assume this fits memory?\n",
      "\n",
      "* Is the graph directed?\n",
      "    * Yes\n",
      "* Can we assume we already have Graph and Node classes?\n",
    "    * Yes\n",
    "* Can we assume the inputs are valid?\n",
    "    * Yes\n",
    "* Can we assume this fits memory?\n",
      "    * Yes"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
keep keep keep add keep keep keep keep keep keep
 Polish graph path exists challenge and solution (#89)

Update constraints and code. 
graphs_trees/graph_path_exists/path_exists_challenge.ipynb