      "\n",
      "## Test Cases\n",
      "\n",
      "### In-Order Traversal\n",
      "\n",
      "* Recursively call in-order traversal on the left child\n",
      "* Visit the current node\n",
      "* Recursively call in-order traversal on the right child\n",
      "\n",
 
</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  </s> remove     "* 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"
 </s> add     "* Space: O(m), where m is the recursion depth, or O(1) if using an iterative approach" </s> remove     "## Problem: Implement depth-first searches (in-order, pre-order, post-order traversals) on a binary tree.\n",
 </s> add     "## Problem: Implement depth-first traversals (in-order, pre-order, post-order) on a binary tree.\n", </s> remove     "## Problem: Implement depth-first searches (in-order, pre-order, post-order traversals) on a binary tree.\n",
 </s> add     "## Problem: Implement depth-first traversals (in-order, pre-order, post-order) on a binary tree.\n",
    "Note:\n",
    "\n",
    "* This following are all forms of depth-first traversals\n",
    "\n",
      "\n",
      "## Test Cases\n",
      "\n",
    "Note:\n",
    "\n",
    "* This following are all forms of depth-first traversals\n",
    "\n",
      "### In-Order Traversal\n",
      "\n",
      "* Recursively call in-order traversal on the left child\n",
      "* Visit the current node\n",
      "* Recursively call in-order traversal on the right child\n",
      "\n",
keep keep add keep keep keep keep keep keep
 Polish tree dfs challenge and solution (#78)

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