diff --git a/README.md b/README.md
index 4ae2272..3daf5d5 100644
--- a/README.md
+++ b/README.md
@@ -209,6 +209,7 @@ Challenges, solutions, and unit tests are presented in the form of **IPython/Jup
| 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) |
| Check if a binary tree is balanced | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/check_balance/check_balance_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/check_balance/check_balance_solution.ipynb) |
| Determine if a tree is a valid binary search tree | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst_validate/bst_validate_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst_validate/bst_validate_solution.ipynb) |
+| Find the in-order successor of a given node in a binary search tree. | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst_successor/bst_successor_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst_successor/bst_successor_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/bst_successor/__init__.py b/graphs_trees/bst_successor/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/graphs_trees/bst_successor/bst_successor_challenge.ipynb b/graphs_trees/bst_successor/bst_successor_challenge.ipynb
new file mode 100644
index 0000000..756019d
--- /dev/null
+++ b/graphs_trees/bst_successor/bst_successor_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: Find the in-order successor of a given node in a binary search 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",
+ "* If there is no successor, do we return None?\n",
+ " * Yes\n",
+ "* Can we assume we already have a Node class that keeps track of parents?\n",
+ " * Yes"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Test Cases\n",
+ "\n",
+ "\n",
+ " _5_\n",
+ " / \\\n",
+ " 3 8\n",
+ " / \\ / \\\n",
+ " 2 4 6 12\n",
+ " / \\ / \\\n",
+ " 1 7 10 15\n",
+ " /\n",
+ " 9\n",
+ "\n",
+ "In: 4 Out: 5\n",
+ "In: 5 Out: 6\n",
+ "In: 8 Out: 9\n",
+ "In: 15 Out: None\n",
+ "
"
+ ]
+ },
+ {
+ "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/bst_successor/bst_successor_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 bst_successor(node):\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_bst_successor.py\n",
+ "from nose.tools import assert_equal\n",
+ "\n",
+ "\n",
+ "class TestBstSuccessor(object):\n",
+ "\n",
+ " def test_bst_successor(self):\n",
+ " nodes = {}\n",
+ " node = Node(5)\n",
+ " nodes[5] = node\n",
+ " nodes[3] = insert(node, 3)\n",
+ " nodes[8] = insert(node, 8)\n",
+ " nodes[2] = insert(node, 2)\n",
+ " nodes[4] = insert(node, 4)\n",
+ " nodes[6] = insert(node, 6)\n",
+ " nodes[12] = insert(node, 12)\n",
+ " nodes[1] = insert(node, 1)\n",
+ " nodes[7] = insert(node, 7)\n",
+ " nodes[10] = insert(node, 10)\n",
+ " nodes[15] = insert(node, 15)\n",
+ " nodes[9] = insert(node, 9)\n",
+ "\n",
+ " assert_equal(bst_successor(nodes[4]), 5)\n",
+ " assert_equal(bst_successor(nodes[5]), 6)\n",
+ " assert_equal(bst_successor(nodes[8]), 9)\n",
+ " assert_equal(bst_successor(nodes[15]), None)\n",
+ "\n",
+ " print('Success: test_bst_successor')\n",
+ "\n",
+ "\n",
+ "def main():\n",
+ " test = TestBstSuccessor()\n",
+ " test.test_bst_successor()\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/bst_successor/bst_successor_solution.ipynb) 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": "3.4.3"
+ }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}
diff --git a/graphs_trees/bst_successor/bst_successor_solution.ipynb b/graphs_trees/bst_successor/bst_successor_solution.ipynb
new file mode 100644
index 0000000..64de61a
--- /dev/null
+++ b/graphs_trees/bst_successor/bst_successor_solution.ipynb
@@ -0,0 +1,234 @@
+{
+ "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: Find the in-order successor of a given node in a binary search 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",
+ "* If there is no successor, do we return None?\n",
+ " * Yes\n",
+ "* Can we assume we already have a Node class that keeps track of parents?\n",
+ " * Yes"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Test Cases\n",
+ "\n",
+ "\n",
+ " _5_\n",
+ " / \\\n",
+ " 3 8\n",
+ " / \\ / \\\n",
+ " 2 4 6 12\n",
+ " / \\ / \\\n",
+ " 1 7 10 15\n",
+ " /\n",
+ " 9\n",
+ "\n",
+ "In: 4 Out: 5\n",
+ "In: 5 Out: 6\n",
+ "In: 8 Out: 9\n",
+ "In: 15 Out: None\n",
+ "
"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Algorithm\n",
+ "\n",
+ "* If the node has a right subtree, return the left-most node in the right subtree\n",
+ "* Else, go up until you find a node that is its parent's left node\n",
+ " * If you get to the root (ie node.parent is None), return None\n",
+ " * Else, return the parent\n",
+ " \n",
+ "Complexity:\n",
+ "* Time: O(n)\n",
+ "* Space: O(h), where h is the height of the tree"
+ ]
+ },
+ {
+ "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 left_most_node(node):\n",
+ " if node is None:\n",
+ " return\n",
+ " if node.left is None:\n",
+ " return node.data\n",
+ " return left_most_node(node.left)\n",
+ "\n",
+ "\n",
+ "def bst_successor(node):\n",
+ " if node is None:\n",
+ " return\n",
+ " if node.right is not None:\n",
+ " return left_most_node(node.right)\n",
+ " else:\n",
+ " parent = node.parent\n",
+ " while parent is not None and parent.left is not node:\n",
+ " node = node.parent\n",
+ " parent = node.parent\n",
+ " if parent is None:\n",
+ " return\n",
+ " return parent.data"
+ ]
+ },
+ {
+ "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_bst_successor.py\n"
+ ]
+ }
+ ],
+ "source": [
+ "%%writefile test_bst_successor.py\n",
+ "from nose.tools import assert_equal\n",
+ "\n",
+ "\n",
+ "class TestBstSuccessor(object):\n",
+ "\n",
+ " def test_bst_successor(self):\n",
+ " nodes = {}\n",
+ " node = Node(5)\n",
+ " nodes[5] = node\n",
+ " nodes[3] = insert(node, 3)\n",
+ " nodes[8] = insert(node, 8)\n",
+ " nodes[2] = insert(node, 2)\n",
+ " nodes[4] = insert(node, 4)\n",
+ " nodes[6] = insert(node, 6)\n",
+ " nodes[12] = insert(node, 12)\n",
+ " nodes[1] = insert(node, 1)\n",
+ " nodes[7] = insert(node, 7)\n",
+ " nodes[10] = insert(node, 10)\n",
+ " nodes[15] = insert(node, 15)\n",
+ " nodes[9] = insert(node, 9)\n",
+ "\n",
+ " assert_equal(bst_successor(nodes[4]), 5)\n",
+ " assert_equal(bst_successor(nodes[5]), 6)\n",
+ " assert_equal(bst_successor(nodes[8]), 9)\n",
+ " assert_equal(bst_successor(nodes[15]), None)\n",
+ "\n",
+ " print('Success: test_bst_successor')\n",
+ "\n",
+ "\n",
+ "def main():\n",
+ " test = TestBstSuccessor()\n",
+ " test.test_bst_successor()\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_bst_successor\n"
+ ]
+ }
+ ],
+ "source": [
+ "%run -i test_bst_successor.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": "3.4.3"
+ }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 0
+}
diff --git a/graphs_trees/bst_successor/test_bst_successor.py b/graphs_trees/bst_successor/test_bst_successor.py
new file mode 100644
index 0000000..2e57be5
--- /dev/null
+++ b/graphs_trees/bst_successor/test_bst_successor.py
@@ -0,0 +1,36 @@
+from nose.tools import assert_equal
+
+
+class TestBstSuccessor(object):
+
+ def test_bst_successor(self):
+ nodes = {}
+ node = Node(5)
+ nodes[5] = node
+ nodes[3] = insert(node, 3)
+ nodes[8] = insert(node, 8)
+ nodes[2] = insert(node, 2)
+ nodes[4] = insert(node, 4)
+ nodes[6] = insert(node, 6)
+ nodes[12] = insert(node, 12)
+ nodes[1] = insert(node, 1)
+ nodes[7] = insert(node, 7)
+ nodes[10] = insert(node, 10)
+ nodes[15] = insert(node, 15)
+ nodes[9] = insert(node, 9)
+
+ assert_equal(bst_successor(nodes[4]), 5)
+ assert_equal(bst_successor(nodes[5]), 6)
+ assert_equal(bst_successor(nodes[8]), 9)
+ assert_equal(bst_successor(nodes[15]), None)
+
+ print('Success: test_bst_successor')
+
+
+def main():
+ test = TestBstSuccessor()
+ test.test_bst_successor()
+
+
+if __name__ == '__main__':
+ main()
\ No newline at end of file