diff --git a/graphs_trees/invert_tree/__init__.py b/graphs_trees/invert_tree/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/graphs_trees/invert_tree/invert_tree_challenge.ipynb b/graphs_trees/invert_tree/invert_tree_challenge.ipynb
new file mode 100644
index 0000000..d701ae8
--- /dev/null
+++ b/graphs_trees/invert_tree/invert_tree_challenge.ipynb
@@ -0,0 +1,200 @@
+{
+ "cells": [
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "This notebook was prepared by [Donne Martin](https://github.com/donnemartin). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges)."
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "# Challenge Notebook"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Problem: Invert a binary tree.\n",
+ "\n",
+ "* [Constraints](#Constraints)\n",
+ "* [Test Cases](#Test-Cases)\n",
+ "* [Algorithm](#Algorithm)\n",
+ "* [Code](#Code)\n",
+ "* [Unit Test](#Unit-Test)\n",
+ "* [Solution Notebook](#Solution-Notebook)"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Constraints\n",
+ "\n",
+ "* What does it mean to invert a binary tree?\n",
+ " * Swap all left and right node pairs\n",
+ "* Can we assume we already have a Node class?\n",
+ " * Yes\n",
+ "* Can we assume the inputs are valid?\n",
+ " * No\n",
+ "* Can we assume this fits memory?\n",
+ " * Yes"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Test Cases\n",
+ "\n",
+ "\n",
+ "Input:\n",
+ " 5\n",
+ " / \\\n",
+ " 2 7\n",
+ " / \\ / \\\n",
+ "1 3 6 9\n",
+ "\n",
+ "Output:\n",
+ " 5\n",
+ " / \\\n",
+ " 7 2\n",
+ " / \\ / \\\n",
+ "9 6 3 1\n",
+ "
"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Algorithm\n",
+ "\n",
+ "Refer to the [Solution Notebook](). If you are stuck and need a hint, the solution notebook's algorithm discussion might be a good place to start."
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Code"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": null,
+ "metadata": {
+ "collapsed": true
+ },
+ "outputs": [],
+ "source": [
+ "%run ../bst/bst.py"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": null,
+ "metadata": {
+ "collapsed": false
+ },
+ "outputs": [],
+ "source": [
+ "class InverseBst(Bst):\n",
+ "\n",
+ " def invert_tree(self):\n",
+ " # TODO: Implement me\n",
+ " pass"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Unit Test"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "**The following unit test is expected to fail until you solve the challenge.**"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": null,
+ "metadata": {
+ "collapsed": false
+ },
+ "outputs": [],
+ "source": [
+ "# %load test_invert_tree.py\n",
+ "from nose.tools import assert_equal\n",
+ "\n",
+ "\n",
+ "class TestInvertTree(object):\n",
+ "\n",
+ " def test_invert_tree(self):\n",
+ " root = Node(5)\n",
+ " bst = InverseBst(root)\n",
+ " node2 = bst.insert(2)\n",
+ " node3 = bst.insert(3)\n",
+ " node1 = bst.insert(1)\n",
+ " node7 = bst.insert(7)\n",
+ " node6 = bst.insert(6)\n",
+ " node9 = bst.insert(9)\n",
+ " result = bst.invert_tree()\n",
+ " assert_equal(result, root)\n",
+ " assert_equal(result.left, node7)\n",
+ " assert_equal(result.right, node2)\n",
+ " assert_equal(result.left.left, node9)\n",
+ " assert_equal(result.left.right, node6)\n",
+ " assert_equal(result.right.left, node3)\n",
+ " assert_equal(result.right.right, node1)\n",
+ " print('Success: test_invert_tree')\n",
+ "\n",
+ "\n",
+ "def main():\n",
+ " test = TestInvertTree()\n",
+ " test.test_invert_tree()\n",
+ "\n",
+ "\n",
+ "if __name__ == '__main__':\n",
+ " main()"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Solution Notebook\n",
+ "\n",
+ "Review the [Solution Notebook]() for a discussion on algorithms and code solutions."
+ ]
+ }
+ ],
+ "metadata": {
+ "kernelspec": {
+ "display_name": "Python 3",
+ "language": "python",
+ "name": "python3"
+ },
+ "language_info": {
+ "codemirror_mode": {
+ "name": "ipython",
+ "version": 3
+ },
+ "file_extension": ".py",
+ "mimetype": "text/x-python",
+ "name": "python",
+ "nbconvert_exporter": "python",
+ "pygments_lexer": "ipython3",
+ "version": "3.5.0"
+ }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}
diff --git a/graphs_trees/invert_tree/invert_tree_solution.ipynb b/graphs_trees/invert_tree/invert_tree_solution.ipynb
new file mode 100644
index 0000000..3037e57
--- /dev/null
+++ b/graphs_trees/invert_tree/invert_tree_solution.ipynb
@@ -0,0 +1,229 @@
+{
+ "cells": [
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "This notebook was prepared by [Donne Martin](https://github.com/donnemartin). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges)."
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "# Solution Notebook"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Problem: Invert a binary tree.\n",
+ "\n",
+ "* [Constraints](#Constraints)\n",
+ "* [Test Cases](#Test-Cases)\n",
+ "* [Algorithm](#Algorithm)\n",
+ "* [Code](#Code)\n",
+ "* [Unit Test](#Unit-Test)"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Constraints\n",
+ "\n",
+ "* What does it mean to invert a binary tree?\n",
+ " * Swap all left and right node pairs\n",
+ "* Can we assume we already have a Node class?\n",
+ " * Yes\n",
+ "* Can we assume the inputs are valid?\n",
+ " * No\n",
+ "* Can we assume this fits memory?\n",
+ " * Yes"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Test Cases\n",
+ "\n",
+ "\n",
+ "Input:\n",
+ " 5\n",
+ " / \\\n",
+ " 2 7\n",
+ " / \\ / \\\n",
+ "1 3 6 9\n",
+ "\n",
+ "Output:\n",
+ " 5\n",
+ " / \\\n",
+ " 7 2\n",
+ " / \\ / \\\n",
+ "9 6 3 1\n",
+ "
"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Algorithm\n",
+ "\n",
+ "* Base case\n",
+ " * If the root is None, return\n",
+ "* Recursive case\n",
+ " * Recurse on the left node\n",
+ " * Recurse on the right node\n",
+ " * Swap left and right\n",
+ "* Return the node\n",
+ "\n",
+ "Complexity:\n",
+ "* Time: O(n)\n",
+ "* Space: O(h), where h is the height, for the recursion depth"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Code"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 1,
+ "metadata": {
+ "collapsed": false
+ },
+ "outputs": [],
+ "source": [
+ "%run ../bst/bst.py"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 2,
+ "metadata": {
+ "collapsed": false
+ },
+ "outputs": [],
+ "source": [
+ "class InverseBst(Bst):\n",
+ "\n",
+ " def invert_tree(self):\n",
+ " if self.root is None:\n",
+ " raise TypeError('root cannot be None')\n",
+ " return self._invert_tree(self.root)\n",
+ "\n",
+ " def _invert_tree(self, root):\n",
+ " if root is None:\n",
+ " return\n",
+ " self._invert_tree(root.left)\n",
+ " self._invert_tree(root.right)\n",
+ " root.left, root.right = root.right, root.left\n",
+ " return root"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Unit Test"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 3,
+ "metadata": {
+ "collapsed": false
+ },
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "Overwriting test_invert_tree.py\n"
+ ]
+ }
+ ],
+ "source": [
+ "%%writefile test_invert_tree.py\n",
+ "from nose.tools import assert_equal, assert_raises\n",
+ "\n",
+ "\n",
+ "class TestInvertTree(object):\n",
+ "\n",
+ " def test_invert_tree(self):\n",
+ " root = Node(5)\n",
+ " bst = InverseBst(root)\n",
+ " node2 = bst.insert(2)\n",
+ " node3 = bst.insert(3)\n",
+ " node1 = bst.insert(1)\n",
+ " node7 = bst.insert(7)\n",
+ " node6 = bst.insert(6)\n",
+ " node9 = bst.insert(9)\n",
+ " result = bst.invert_tree()\n",
+ " assert_equal(result, root)\n",
+ " assert_equal(result.left, node7)\n",
+ " assert_equal(result.right, node2)\n",
+ " assert_equal(result.left.left, node9)\n",
+ " assert_equal(result.left.right, node6)\n",
+ " assert_equal(result.right.left, node3)\n",
+ " assert_equal(result.right.right, node1)\n",
+ " print('Success: test_invert_tree')\n",
+ "\n",
+ "\n",
+ "def main():\n",
+ " test = TestInvertTree()\n",
+ " test.test_invert_tree()\n",
+ "\n",
+ "\n",
+ "if __name__ == '__main__':\n",
+ " main()"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 4,
+ "metadata": {
+ "collapsed": false
+ },
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "Success: test_invert_tree\n"
+ ]
+ }
+ ],
+ "source": [
+ "%run -i test_invert_tree.py"
+ ]
+ }
+ ],
+ "metadata": {
+ "kernelspec": {
+ "display_name": "Python 3",
+ "language": "python",
+ "name": "python3"
+ },
+ "language_info": {
+ "codemirror_mode": {
+ "name": "ipython",
+ "version": 3
+ },
+ "file_extension": ".py",
+ "mimetype": "text/x-python",
+ "name": "python",
+ "nbconvert_exporter": "python",
+ "pygments_lexer": "ipython3",
+ "version": "3.5.0"
+ }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}
diff --git a/graphs_trees/invert_tree/test_invert_tree.py b/graphs_trees/invert_tree/test_invert_tree.py
new file mode 100644
index 0000000..ab33458
--- /dev/null
+++ b/graphs_trees/invert_tree/test_invert_tree.py
@@ -0,0 +1,32 @@
+from nose.tools import assert_equal, assert_raises
+
+
+class TestInvertTree(object):
+
+ def test_invert_tree(self):
+ root = Node(5)
+ bst = InverseBst(root)
+ node2 = bst.insert(2)
+ node3 = bst.insert(3)
+ node1 = bst.insert(1)
+ node7 = bst.insert(7)
+ node6 = bst.insert(6)
+ node9 = bst.insert(9)
+ result = bst.invert_tree()
+ assert_equal(result, root)
+ assert_equal(result.left, node7)
+ assert_equal(result.right, node2)
+ assert_equal(result.left.left, node9)
+ assert_equal(result.left.right, node6)
+ assert_equal(result.right.left, node3)
+ assert_equal(result.right.right, node1)
+ print('Success: test_invert_tree')
+
+
+def main():
+ test = TestInvertTree()
+ test.test_invert_tree()
+
+
+if __name__ == '__main__':
+ main()
\ No newline at end of file