      "    queue.append(start)\n",
      "    start.visit_state = State.visited\n",
      "    while queue:\n",
      "        node = queue.popleft()\n",
      "        if node is None:\n",
      "            continue\n",
      "        if node is end:\n",
      "            return True\n",
      "        for adj_node in node.adjacent:\n",
      "            if adj_node.visit_state == State.unvisited:\n",
      "                queue.append(adj_node)\n",
 
</s> remove    "pygments_lexer": "ipython2",
   "version": "2.7.10"
 </s> add    "pygments_lexer": "ipython3",
   "version": "3.5.0" </s> remove     "version": 2
 </s> add     "version": 3 </s> remove    "name": "python2"
 </s> add    "name": "python3"
      "    queue.append(start)\n",
      "    start.visit_state = State.visited\n",
      "    while queue:\n",
      "        node = queue.popleft()\n",


      "        if node is end:\n",
      "            return True\n",
      "        for adj_node in node.adjacent:\n",
      "            if adj_node.visit_state == State.unvisited:\n",
      "                queue.append(adj_node)\n",
keep keep keep keep replace replace keep keep keep keep keep
 Polish graph path exists challenge and solution (#89)

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