    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## Problem: Implement depth-first searches (in-order, pre-order, post-order traversals) on a binary tree.\n",
      "\n",
      "* [Constraints](#Constraints)\n",
      "* [Test Cases](#Test-Cases)\n",
      "* [Algorithm](#Algorithm)\n",
      "* [Code](#Code)\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", </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"
    "## Problem: Implement depth-first traversals (in-order, pre-order, post-order) on a binary tree.\n",
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
    "## Problem: Implement depth-first traversals (in-order, pre-order, post-order) on a binary tree.\n",
      "\n",
      "* [Constraints](#Constraints)\n",
      "* [Test Cases](#Test-Cases)\n",
      "* [Algorithm](#Algorithm)\n",
      "* [Code](#Code)\n",
keep keep keep keep replace keep keep keep keep keep
 Polish tree dfs challenge and solution (#78)

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