diff --git a/README.md b/README.md
index 6b6fb24..2d3d96b 100644
--- a/README.md
+++ b/README.md
@@ -207,6 +207,7 @@ Challenges, solutions, and unit tests are presented in the form of **IPython/Jup
| Implement breadth-first search on a tree | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/tree_bfs/bfs_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/tree_bfs/bfs_solution.ipynb) |
| Determine the height of a tree | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/tree_height/height_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/tree_height/height_solution.ipynb) |
| Create a binary search tree with minimal height from a sorted array | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst_min/bst_min_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst_min/bst_min_solution.ipynb) |
+| Create a linked list for each level of a binary tree | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/tree_level_lists/tree_level_lists_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/tree_level_lists/tree_level_lists_solution.ipynb) |
| Implement a binary search tree | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst/bst_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst/bst_solution.ipynb) |
| Implement depth-first search on a graph | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/graph_dfs/dfs_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/graph_dfs/dfs_solution.ipynb) |
| Implement breadth-first search on a graph | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/graph_bfs/bfs_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/graph_bfs/bfs_solution.ipynb) |
diff --git a/graphs_trees/tree_level_lists/__init__.py b/graphs_trees/tree_level_lists/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/graphs_trees/tree_level_lists/test_tree_level_lists.py b/graphs_trees/tree_level_lists/test_tree_level_lists.py
new file mode 100644
index 0000000..9a9b4b2
--- /dev/null
+++ b/graphs_trees/tree_level_lists/test_tree_level_lists.py
@@ -0,0 +1,42 @@
+from nose.tools import assert_equal
+
+
+class TestTreeLevelLists(object):
+
+ def test_tree_level_lists(self):
+ node = Node(5)
+ insert(node, 3)
+ insert(node, 8)
+ insert(node, 2)
+ insert(node, 4)
+ insert(node, 1)
+ insert(node, 7)
+ insert(node, 6)
+ insert(node, 9)
+ insert(node, 10)
+ insert(node, 11)
+
+ levels = create_level_lists(node)
+ results_list = []
+ for level in levels:
+ results = Results()
+ for node in level:
+ results.add_result(node)
+ results_list.append(results)
+
+ assert_equal(str(results_list[0]), '[5]')
+ assert_equal(str(results_list[1]), '[3, 8]')
+ assert_equal(str(results_list[2]), '[2, 4, 7, 9]')
+ assert_equal(str(results_list[3]), '[1, 6, 10]')
+ assert_equal(str(results_list[4]), '[11]')
+
+ print('Success: test_tree_level_lists')
+
+
+def main():
+ test = TestTreeLevelLists()
+ test.test_tree_level_lists()
+
+
+if __name__ == '__main__':
+ main()
\ No newline at end of file
diff --git a/graphs_trees/tree_level_lists/tree_level_lists_challenge.ipynb b/graphs_trees/tree_level_lists/tree_level_lists_challenge.ipynb
new file mode 100644
index 0000000..ba7ff1c
--- /dev/null
+++ b/graphs_trees/tree_level_lists/tree_level_lists_challenge.ipynb
@@ -0,0 +1,202 @@
+{
+ "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: Create a linked list for each level of 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",
+ "* Is this a binary search tree?\n",
+ " * Yes\n",
+ "* Can we assume we already have a Node class with an insert method?\n",
+ " * Yes"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Test Cases\n",
+ "\n",
+ "* 5, 3, 8, 2, 4, 1, 7, 6, 9, 10, 11 -> [[5], [3, 8], [2, 4, 7, 9], [1, 6, 10], [11]]"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Algorithm\n",
+ "\n",
+ "Refer to the [Solution Notebook](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/tree_level_lists/tree_level_lists_solution.ipynb). 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\n",
+ "%load ../bst/bst.py"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": null,
+ "metadata": {
+ "collapsed": false
+ },
+ "outputs": [],
+ "source": [
+ "def create_level_lists(root):\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": true
+ },
+ "outputs": [],
+ "source": [
+ "%run ../utils/results.py"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": null,
+ "metadata": {
+ "collapsed": false
+ },
+ "outputs": [],
+ "source": [
+ "# %load test_tree_level_lists.py\n",
+ "from nose.tools import assert_equal\n",
+ "\n",
+ "\n",
+ "class TestTreeLevelLists(object):\n",
+ "\n",
+ " def test_tree_level_lists(self):\n",
+ " node = Node(5)\n",
+ " insert(node, 3)\n",
+ " insert(node, 8)\n",
+ " insert(node, 2)\n",
+ " insert(node, 4)\n",
+ " insert(node, 1)\n",
+ " insert(node, 7)\n",
+ " insert(node, 6)\n",
+ " insert(node, 9)\n",
+ " insert(node, 10)\n",
+ " insert(node, 11)\n",
+ "\n",
+ " levels = create_level_lists(node)\n",
+ " results_list = []\n",
+ " for level in levels:\n",
+ " results = Results()\n",
+ " for node in level:\n",
+ " results.add_result(node)\n",
+ " results_list.append(results)\n",
+ "\n",
+ " assert_equal(str(results_list[0]), '[5]')\n",
+ " assert_equal(str(results_list[1]), '[3, 8]')\n",
+ " assert_equal(str(results_list[2]), '[2, 4, 7, 9]')\n",
+ " assert_equal(str(results_list[3]), '[1, 6, 10]')\n",
+ " assert_equal(str(results_list[4]), '[11]')\n",
+ "\n",
+ " print('Success: test_tree_level_lists')\n",
+ "\n",
+ "\n",
+ "def main():\n",
+ " test = TestTreeLevelLists()\n",
+ " test.test_tree_level_lists()\n",
+ "\n",
+ "\n",
+ "if __name__ == '__main__':\n",
+ " main()"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Solution Notebook\n",
+ "\n",
+ "Review the [Solution Notebook](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/tree_level_lists/tree_level_lists_solution.ipynb) for a discussion on algorithms and code solutions."
+ ]
+ }
+ ],
+ "metadata": {
+ "kernelspec": {
+ "display_name": "Python 2",
+ "language": "python",
+ "name": "python2"
+ },
+ "language_info": {
+ "codemirror_mode": {
+ "name": "ipython",
+ "version": 2
+ },
+ "file_extension": ".py",
+ "mimetype": "text/x-python",
+ "name": "python",
+ "nbconvert_exporter": "python",
+ "pygments_lexer": "ipython2",
+ "version": "2.7.10"
+ }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}
diff --git a/graphs_trees/tree_level_lists/tree_level_lists_solution.ipynb b/graphs_trees/tree_level_lists/tree_level_lists_solution.ipynb
new file mode 100644
index 0000000..dbfc106
--- /dev/null
+++ b/graphs_trees/tree_level_lists/tree_level_lists_solution.ipynb
@@ -0,0 +1,239 @@
+{
+ "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: Create a linked list for each level of 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",
+ "* Is this a binary search tree?\n",
+ " * Yes\n",
+ "* Can we assume we already have a Node class with an insert method?\n",
+ " * Yes"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Test Cases\n",
+ "\n",
+ "* 5, 3, 8, 2, 4, 1, 7, 6, 9, 10, 11 -> [[5], [3, 8], [2, 4, 7, 9], [1, 6, 10], [11]]"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Algorithm\n",
+ "\n",
+ "We can use either a depth-first or a breadth-first search. Intuitively, it seems like a breadth-first search might be a better fit as we are creating a linked list for each level.\n",
+ "\n",
+ "We can use a modified breadth-first search that keeps track of parents as we build the linked list for the current level.\n",
+ "\n",
+ "* Append the root to the current level's linked list `current`\n",
+ "* While the `current` is not empty:\n",
+ " * Add `current` to `results`\n",
+ " * Set `parents` to `current` to prepare to go one level deeper\n",
+ " * Clear `current` so it can hold the next level\n",
+ " * For each `parent` in `parents`, add the children to `current`\n",
+ "* Return the results\n",
+ " \n",
+ "Complexity:\n",
+ "* Time: O(n)\n",
+ "* Space: O(n)"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Code"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 1,
+ "metadata": {
+ "collapsed": true
+ },
+ "outputs": [],
+ "source": [
+ "%run ../bst/bst.py"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 2,
+ "metadata": {
+ "collapsed": false
+ },
+ "outputs": [],
+ "source": [
+ "def create_level_lists(root):\n",
+ " if root is None:\n",
+ " return\n",
+ " results = []\n",
+ " current = []\n",
+ " parents = []\n",
+ " current.append(root)\n",
+ " while current:\n",
+ " results.append(current)\n",
+ " parents = list(current)\n",
+ " current = []\n",
+ " for parent in parents:\n",
+ " if parent.left is not None:\n",
+ " current.append(parent.left)\n",
+ " if parent.right is not None:\n",
+ " current.append(parent.right)\n",
+ " return results"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Unit Test"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 3,
+ "metadata": {
+ "collapsed": true
+ },
+ "outputs": [],
+ "source": [
+ "%run ../utils/results.py"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 4,
+ "metadata": {
+ "collapsed": false
+ },
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "Overwriting test_tree_level_lists.py\n"
+ ]
+ }
+ ],
+ "source": [
+ "%%writefile test_tree_level_lists.py\n",
+ "from nose.tools import assert_equal\n",
+ "\n",
+ "\n",
+ "class TestTreeLevelLists(object):\n",
+ "\n",
+ " def test_tree_level_lists(self):\n",
+ " node = Node(5)\n",
+ " insert(node, 3)\n",
+ " insert(node, 8)\n",
+ " insert(node, 2)\n",
+ " insert(node, 4)\n",
+ " insert(node, 1)\n",
+ " insert(node, 7)\n",
+ " insert(node, 6)\n",
+ " insert(node, 9)\n",
+ " insert(node, 10)\n",
+ " insert(node, 11)\n",
+ "\n",
+ " levels = create_level_lists(node)\n",
+ " results_list = []\n",
+ " for level in levels:\n",
+ " results = Results()\n",
+ " for node in level:\n",
+ " results.add_result(node)\n",
+ " results_list.append(results)\n",
+ "\n",
+ " assert_equal(str(results_list[0]), '[5]')\n",
+ " assert_equal(str(results_list[1]), '[3, 8]')\n",
+ " assert_equal(str(results_list[2]), '[2, 4, 7, 9]')\n",
+ " assert_equal(str(results_list[3]), '[1, 6, 10]')\n",
+ " assert_equal(str(results_list[4]), '[11]')\n",
+ "\n",
+ " print('Success: test_tree_level_lists')\n",
+ "\n",
+ "\n",
+ "def main():\n",
+ " test = TestTreeLevelLists()\n",
+ " test.test_tree_level_lists()\n",
+ "\n",
+ "\n",
+ "if __name__ == '__main__':\n",
+ " main()"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 5,
+ "metadata": {
+ "collapsed": false
+ },
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "Success: test_tree_level_lists\n"
+ ]
+ }
+ ],
+ "source": [
+ "%run -i test_tree_level_lists.py"
+ ]
+ }
+ ],
+ "metadata": {
+ "kernelspec": {
+ "display_name": "Python 2",
+ "language": "python",
+ "name": "python2"
+ },
+ "language_info": {
+ "codemirror_mode": {
+ "name": "ipython",
+ "version": 2
+ },
+ "file_extension": ".py",
+ "mimetype": "text/x-python",
+ "name": "python",
+ "nbconvert_exporter": "python",
+ "pygments_lexer": "ipython2",
+ "version": "2.7.10"
+ }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}