diff --git a/README.md b/README.md
index c1be112..25cbf71 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 depth-first search (pre-, in-, post-order) | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/tree_dfs/dfs_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/tree_dfs/dfs_solution.ipynb) |
| Implement breadth-first search | [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) |
+| Implement a basic graph | [Challenge](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/graph/graph_challenge.ipynb)│[Solution](http://nbviewer.ipython.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/graph/graph_solution.ipynb) |
| Print a tree using pre-order traversal without recursion | [Contribute](https://github.com/donnemartin/interactive-coding-challenges/blob/master/CONTRIBUTING.md)│[Contribute](https://github.com/donnemartin/interactive-coding-challenges/blob/master/CONTRIBUTING.md) |
| Determine the lowest common ancestor of two nodes | [Contribute](https://github.com/donnemartin/interactive-coding-challenges/blob/master/CONTRIBUTING.md)│[Contribute](https://github.com/donnemartin/interactive-coding-challenges/blob/master/CONTRIBUTING.md) |
| Transform a binary tree into a heap | [Contribute](https://github.com/donnemartin/interactive-coding-challenges/blob/master/CONTRIBUTING.md)│[Contribute](https://github.com/donnemartin/interactive-coding-challenges/blob/master/CONTRIBUTING.md) |
diff --git a/graphs_trees/graph/__init__.py b/graphs_trees/graph/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/graphs_trees/graph/graph.py b/graphs_trees/graph/graph.py
new file mode 100644
index 0000000..614f997
--- /dev/null
+++ b/graphs_trees/graph/graph.py
@@ -0,0 +1,34 @@
+# Python 2 users: Run pip install enum34
+from enum import Enum
+
+
+class State(Enum):
+ unvisited = 1
+ visited = 2
+ visiting = 3
+
+
+class Node:
+
+ def __init__(self, id):
+ self.id = id
+ self.state = State.unvisited
+ self.connections = {}
+
+
+class Graph:
+
+ def __init__(self):
+ self.nodes = {}
+
+ def add_node(self, id):
+ node = Node(id)
+ self.nodes[id] = node
+ return node
+
+ def add_edge(self, source, dest, weight=0):
+ if source not in self.nodes:
+ self.add_node(source)
+ if dest not in self.nodes:
+ self.add_node(dest)
+ self.nodes[source].connections[self.nodes[dest]] = weight
diff --git a/graphs_trees/graph/graph_challenge.ipynb b/graphs_trees/graph/graph_challenge.ipynb
new file mode 100644
index 0000000..dd08de2
--- /dev/null
+++ b/graphs_trees/graph/graph_challenge.ipynb
@@ -0,0 +1,217 @@
+{
+ "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: Implement a basic graph.\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",
+ "* I plan to use a Node and Graph class. \n",
+ " * Node will keep track of its id, visit state, and connections. \n",
+ " * Connections will be a dictionary with the key = node id and value being the edge weight\n",
+ " * Graph will hold all of the nodes and have functions to add nodes and edges. \n",
+ " * Is this in line with what you have in mind?\n",
+ " * Yes"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Test Cases\n",
+ "\n",
+ "Input:\n",
+ "* `add_edge(source, destination, weight)`\n",
+ "\n",
+ "```\n",
+ "graph.add_edge(0, 1, 5)\n",
+ "graph.add_edge(0, 5, 2)\n",
+ "graph.add_edge(1, 2, 3)\n",
+ "graph.add_edge(2, 3, 4)\n",
+ "graph.add_edge(3, 4, 5)\n",
+ "graph.add_edge(3, 5, 6)\n",
+ "graph.add_edge(4, 0, 7)\n",
+ "graph.add_edge(5, 4, 8)\n",
+ "graph.add_edge(5, 2, 9)\n",
+ "```\n",
+ "\n",
+ "Result:\n",
+ "* `source` and `destination` nodes within `graph` are connected with specified `weight`."
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Algorithm\n",
+ "\n",
+ "Refer to the [Solution Notebook](https://github.com/donnemartin/interactive-coding-challenges/graphs_trees/graphs/graph_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": false
+ },
+ "outputs": [],
+ "source": [
+ "# Python 2 users: Run pip install enum34\n",
+ "from enum import Enum\n",
+ "\n",
+ "\n",
+ "class State(Enum):\n",
+ " unvisited = 1\n",
+ " visited = 2\n",
+ " visiting = 3\n",
+ "\n",
+ "\n",
+ "class Node:\n",
+ "\n",
+ " def __init__(self, id):\n",
+ " # TODO: Implement me\n",
+ "\n",
+ "\n",
+ "class Graph:\n",
+ "\n",
+ " def __init__(self):\n",
+ " # TODO: Implement me\n",
+ "\n",
+ " def add_node(self, id):\n",
+ " # TODO: Implement me\n",
+ "\n",
+ " def add_edge(self, source, dest, weight=0):\n",
+ " # TODO: Implement me"
+ ]
+ },
+ {
+ "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_graph.py\n",
+ "from nose.tools import assert_equal\n",
+ "\n",
+ "\n",
+ "class TestGraph(object):\n",
+ "\n",
+ " def test_graph(self):\n",
+ " graph = Graph()\n",
+ " for key in range(0, 6):\n",
+ " graph.add_node(key)\n",
+ " graph.add_edge(0, 1, 5)\n",
+ " graph.add_edge(0, 5, 2)\n",
+ " graph.add_edge(1, 2, 3)\n",
+ " graph.add_edge(2, 3, 4)\n",
+ " graph.add_edge(3, 4, 5)\n",
+ " graph.add_edge(3, 5, 6)\n",
+ " graph.add_edge(4, 0, 7)\n",
+ " graph.add_edge(5, 4, 8)\n",
+ " graph.add_edge(5, 2, 9)\n",
+ "\n",
+ " assert_equal(graph.nodes[0].connections[graph.nodes[1]], 5)\n",
+ " assert_equal(graph.nodes[0].connections[graph.nodes[5]], 2)\n",
+ " assert_equal(graph.nodes[1].connections[graph.nodes[2]], 3)\n",
+ " assert_equal(graph.nodes[2].connections[graph.nodes[3]], 4)\n",
+ " assert_equal(graph.nodes[3].connections[graph.nodes[4]], 5)\n",
+ " assert_equal(graph.nodes[3].connections[graph.nodes[5]], 6)\n",
+ " assert_equal(graph.nodes[4].connections[graph.nodes[0]], 7)\n",
+ " assert_equal(graph.nodes[5].connections[graph.nodes[4]], 8)\n",
+ " assert_equal(graph.nodes[5].connections[graph.nodes[2]], 9)\n",
+ "\n",
+ " print('Success: test_graph')\n",
+ "\n",
+ "\n",
+ "def main():\n",
+ " test = TestGraph()\n",
+ " test.test_graph()\n",
+ "\n",
+ "\n",
+ "if __name__ == '__main__':\n",
+ " main()"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Solution Notebook\n",
+ "\n",
+ "Review the [Solution Notebook](https://github.com/donnemartin/interactive-coding-challenges/graphs_trees/graphs/graph_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/graph/graph_solution.ipynb b/graphs_trees/graph/graph_solution.ipynb
new file mode 100644
index 0000000..acb6055
--- /dev/null
+++ b/graphs_trees/graph/graph_solution.ipynb
@@ -0,0 +1,252 @@
+{
+ "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: Implement a basic graph.\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",
+ "* I plan to use a Node and Graph class. \n",
+ " * Node will keep track of its id, visit state, and connections. \n",
+ " * Connections will be a dictionary with the key = node id and value being the edge weight\n",
+ " * Graph will hold all of the nodes and have functions to add nodes and edges. \n",
+ " * Is this in line with what you have in mind?\n",
+ " * Yes"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Test Cases\n",
+ "\n",
+ "Input:\n",
+ "* `add_edge(source, destination, weight)`\n",
+ "\n",
+ "```\n",
+ "graph.add_edge(0, 1, 5)\n",
+ "graph.add_edge(0, 5, 2)\n",
+ "graph.add_edge(1, 2, 3)\n",
+ "graph.add_edge(2, 3, 4)\n",
+ "graph.add_edge(3, 4, 5)\n",
+ "graph.add_edge(3, 5, 6)\n",
+ "graph.add_edge(4, 0, 7)\n",
+ "graph.add_edge(5, 4, 8)\n",
+ "graph.add_edge(5, 2, 9)\n",
+ "```\n",
+ "\n",
+ "Result:\n",
+ "* `source` and `destination` nodes within `graph` are connected with specified `weight`."
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Algorithm\n",
+ "\n",
+ "### add_node\n",
+ "\n",
+ "* Create a node with the input id\n",
+ "* Add the newly created node to the list of nodes\n",
+ "\n",
+ "Complexity:\n",
+ "* Time: O(1)\n",
+ "* Space: O(1)\n",
+ "\n",
+ "### add_edge\n",
+ "\n",
+ "* If the source node is not in the list of nodes, add it\n",
+ "* If the dest node is not in the list of nodes, add it\n",
+ "* Add a connection from the source node to the dest node with the given edge weight\n",
+ "\n",
+ "Complexity:\n",
+ "* Time: O(1)\n",
+ "* Space: O(1)"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Code"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 1,
+ "metadata": {
+ "collapsed": true
+ },
+ "outputs": [],
+ "source": [
+ "# Python 2 users: Run pip install enum34\n",
+ "from enum import Enum\n",
+ "\n",
+ "\n",
+ "class State(Enum):\n",
+ " unvisited = 1\n",
+ " visited = 2\n",
+ " visiting = 3\n",
+ "\n",
+ "\n",
+ "class Node:\n",
+ "\n",
+ " def __init__(self, id):\n",
+ " self.id = id\n",
+ " self.state = State.unvisited\n",
+ " self.connections = {}\n",
+ "\n",
+ "\n",
+ "class Graph:\n",
+ "\n",
+ " def __init__(self):\n",
+ " self.nodes = {}\n",
+ "\n",
+ " def add_node(self, id):\n",
+ " node = Node(id)\n",
+ " self.nodes[id] = node\n",
+ " return node\n",
+ "\n",
+ " def add_edge(self, source, dest, weight=0):\n",
+ " if source not in self.nodes:\n",
+ " self.add_node(source)\n",
+ " if dest not in self.nodes:\n",
+ " self.add_node(dest)\n",
+ " self.nodes[source].connections[self.nodes[dest]] = weight"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Unit Test"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 2,
+ "metadata": {
+ "collapsed": false
+ },
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "Overwriting test_graph.py\n"
+ ]
+ }
+ ],
+ "source": [
+ "%%writefile test_graph.py\n",
+ "from nose.tools import assert_equal\n",
+ "\n",
+ "\n",
+ "class TestGraph(object):\n",
+ "\n",
+ " def test_graph(self):\n",
+ " graph = Graph()\n",
+ " for key in range(0, 6):\n",
+ " graph.add_node(key)\n",
+ " graph.add_edge(0, 1, 5)\n",
+ " graph.add_edge(0, 5, 2)\n",
+ " graph.add_edge(1, 2, 3)\n",
+ " graph.add_edge(2, 3, 4)\n",
+ " graph.add_edge(3, 4, 5)\n",
+ " graph.add_edge(3, 5, 6)\n",
+ " graph.add_edge(4, 0, 7)\n",
+ " graph.add_edge(5, 4, 8)\n",
+ " graph.add_edge(5, 2, 9)\n",
+ "\n",
+ " assert_equal(graph.nodes[0].connections[graph.nodes[1]], 5)\n",
+ " assert_equal(graph.nodes[0].connections[graph.nodes[5]], 2)\n",
+ " assert_equal(graph.nodes[1].connections[graph.nodes[2]], 3)\n",
+ " assert_equal(graph.nodes[2].connections[graph.nodes[3]], 4)\n",
+ " assert_equal(graph.nodes[3].connections[graph.nodes[4]], 5)\n",
+ " assert_equal(graph.nodes[3].connections[graph.nodes[5]], 6)\n",
+ " assert_equal(graph.nodes[4].connections[graph.nodes[0]], 7)\n",
+ " assert_equal(graph.nodes[5].connections[graph.nodes[4]], 8)\n",
+ " assert_equal(graph.nodes[5].connections[graph.nodes[2]], 9)\n",
+ "\n",
+ " print('Success: test_graph')\n",
+ "\n",
+ "\n",
+ "def main():\n",
+ " test = TestGraph()\n",
+ " test.test_graph()\n",
+ "\n",
+ "\n",
+ "if __name__ == '__main__':\n",
+ " main()"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 3,
+ "metadata": {
+ "collapsed": false
+ },
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "Success: test_graph\n"
+ ]
+ }
+ ],
+ "source": [
+ "%run -i test_graph.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
+}
diff --git a/graphs_trees/graph/test_graph.py b/graphs_trees/graph/test_graph.py
new file mode 100644
index 0000000..44d4b84
--- /dev/null
+++ b/graphs_trees/graph/test_graph.py
@@ -0,0 +1,39 @@
+from nose.tools import assert_equal
+
+
+class TestGraph(object):
+
+ def test_graph(self):
+ graph = Graph()
+ for key in range(0, 6):
+ graph.add_node(key)
+ graph.add_edge(0, 1, 5)
+ graph.add_edge(0, 5, 2)
+ graph.add_edge(1, 2, 3)
+ graph.add_edge(2, 3, 4)
+ graph.add_edge(3, 4, 5)
+ graph.add_edge(3, 5, 6)
+ graph.add_edge(4, 0, 7)
+ graph.add_edge(5, 4, 8)
+ graph.add_edge(5, 2, 9)
+
+ assert_equal(graph.nodes[0].connections[graph.nodes[1]], 5)
+ assert_equal(graph.nodes[0].connections[graph.nodes[5]], 2)
+ assert_equal(graph.nodes[1].connections[graph.nodes[2]], 3)
+ assert_equal(graph.nodes[2].connections[graph.nodes[3]], 4)
+ assert_equal(graph.nodes[3].connections[graph.nodes[4]], 5)
+ assert_equal(graph.nodes[3].connections[graph.nodes[5]], 6)
+ assert_equal(graph.nodes[4].connections[graph.nodes[0]], 7)
+ assert_equal(graph.nodes[5].connections[graph.nodes[4]], 8)
+ assert_equal(graph.nodes[5].connections[graph.nodes[2]], 9)
+
+ print('Success: test_graph')
+
+
+def main():
+ test = TestGraph()
+ test.test_graph()
+
+
+if __name__ == '__main__':
+ main()
\ No newline at end of file