      "* Visit the current node\n",
      "\n",
      "Complexity:\n",
      "* Time: O(n)\n",
      "* Space: O(m), where m is the recursion depth, or O(1) if using an iterative approach\n",
      "\n",
      "Note:\n",
      "* This is a form of a depth-first traversal"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
 
</s> remove     "Note:\n",
    "* This is a form of a depth-first traversal\n",
    "\n",
 </s> add  </s> remove     "Note:\n",
    "* This is a form of a depth-first traversal\n",
    "\n",
 </s> add 
    "* Space: O(m), where m is the recursion depth, or O(1) if using an iterative approach"
      "* Visit the current node\n",
      "\n",
      "Complexity:\n",
      "* Time: O(n)\n",
    "* Space: O(m), where m is the recursion depth, or O(1) if using an iterative approach"
    "* Space: O(m), where m is the recursion depth, or O(1) if using an iterative approach"
    "* Space: O(m), where m is the recursion depth, or O(1) if using an iterative approach"
    "* Space: O(m), where m is the recursion depth, or O(1) if using an iterative approach"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
keep keep keep keep replace replace replace replace keep keep keep keep keep
 Polish tree dfs challenge and solution (#78)

Update constraints and algorithm discussion. 
graphs_trees/tree_dfs/dfs_solution.ipynb