diff --git a/recursion_dynamic/grid_path/__init__.py b/recursion_dynamic/grid_path/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/recursion_dynamic/grid_path/grid_path_challenge.ipynb b/recursion_dynamic/grid_path/grid_path_challenge.ipynb
new file mode 100644
index 0000000..d9139a5
--- /dev/null
+++ b/recursion_dynamic/grid_path/grid_path_challenge.ipynb
@@ -0,0 +1,212 @@
+{
+ "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 an algorithm to have a robot move from the upper left corner to the bottom right corner of a grid.\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",
+ "* Are there restrictions to how the robot moves?\n",
+ " * The robot can only move right and down\n",
+ "* Are some cells off limits?\n",
+ " * Yes\n",
+ "* Is this a rectangular grid? i.e. the grid is not jagged?\n",
+ " * Yes\n",
+ "* Will there always be a valid way for the robot to get to the bottom right?\n",
+ " * No, return None\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",
+ "o = valid cell\n",
+ "x = invalid cell\n",
+ "\n",
+ " 0 1 2 3\n",
+ "0 o o o o\n",
+ "1 o x o o\n",
+ "2 o o x o\n",
+ "3 x o o o\n",
+ "4 o o x o\n",
+ "5 o o o x\n",
+ "6 o x o x\n",
+ "7 o x o o\n",
+ "
\n",
+ "\n",
+ "* General case\n",
+ "\n",
+ "```\n",
+ "expected = [(0, 0), (1, 0), (2, 0),\n",
+ " (2, 1), (3, 1), (4, 1),\n",
+ " (5, 1), (5, 2), (6, 2), \n",
+ " (7, 2), (7, 3)]\n",
+ "```\n",
+ "\n",
+ "* No valid path: In above example, row 7 col 2 is also invalid -> None\n",
+ "* None input -> None\n",
+ "* Empty matrix -> None"
+ ]
+ },
+ {
+ "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": false
+ },
+ "outputs": [],
+ "source": [
+ "class Grid(object):\n",
+ "\n",
+ " def find_path(self, matrix):\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_grid_path.py\n",
+ "from nose.tools import assert_equal\n",
+ "\n",
+ "\n",
+ "class TestGridPath(object):\n",
+ "\n",
+ " def test_grid_path(self):\n",
+ " grid = Grid()\n",
+ " assert_equal(grid.find_path(None), None)\n",
+ " assert_equal(grid.find_path([[]]), None)\n",
+ " max_rows = 8\n",
+ " max_cols = 4\n",
+ " matrix = [[1] * max_cols for _ in range(max_rows)]\n",
+ " matrix[1][1] = 0\n",
+ " matrix[2][2] = 0\n",
+ " matrix[3][0] = 0\n",
+ " matrix[4][2] = 0\n",
+ " matrix[5][3] = 0\n",
+ " matrix[6][1] = 0\n",
+ " matrix[6][3] = 0\n",
+ " matrix[7][1] = 0\n",
+ " result = grid.find_path(matrix)\n",
+ " expected = [(0, 0), (1, 0), (2, 0),\n",
+ " (2, 1), (3, 1), (4, 1),\n",
+ " (5, 1), (5, 2), (6, 2), \n",
+ " (7, 2), (7, 3)]\n",
+ " assert_equal(result, expected)\n",
+ " matrix[7][2] = 0\n",
+ " result = grid.find_path(matrix)\n",
+ " assert_equal(result, None)\n",
+ " print('Success: test_grid_path')\n",
+ "\n",
+ "\n",
+ "def main():\n",
+ " test = TestGridPath()\n",
+ " test.test_grid_path()\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/recursion_dynamic/grid_path/grid_path_solution.ipynb b/recursion_dynamic/grid_path/grid_path_solution.ipynb
new file mode 100644
index 0000000..18fc1f8
--- /dev/null
+++ b/recursion_dynamic/grid_path/grid_path_solution.ipynb
@@ -0,0 +1,276 @@
+{
+ "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 an algorithm to have a robot move from the upper left corner to the bottom right corner of a grid.\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",
+ "* Are there restrictions to how the robot moves?\n",
+ " * The robot can only move right and down\n",
+ "* Are some cells invalid (off limits)?\n",
+ " * Yes\n",
+ "* Can we assume the starting and ending cells are valid cells?\n",
+ " * Yes\n",
+ "* Is this a rectangular grid? i.e. the grid is not jagged?\n",
+ " * Yes\n",
+ "* Will there always be a valid way for the robot to get to the bottom right?\n",
+ " * No, return None\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",
+ "o = valid cell\n",
+ "x = invalid cell\n",
+ "\n",
+ " 0 1 2 3\n",
+ "0 o o o o\n",
+ "1 o x o o\n",
+ "2 o o x o\n",
+ "3 x o o o\n",
+ "4 o o x o\n",
+ "5 o o o x\n",
+ "6 o x o x\n",
+ "7 o x o o\n",
+ "
\n",
+ "\n",
+ "* General case\n",
+ "\n",
+ "```\n",
+ "expected = [(0, 0), (1, 0), (2, 0),\n",
+ " (2, 1), (3, 1), (4, 1),\n",
+ " (5, 1), (5, 2), (6, 2), \n",
+ " (7, 2), (7, 3)]\n",
+ "```\n",
+ "\n",
+ "* No valid path, say row 7, col 2 is invalid\n",
+ "* None input\n",
+ "* Empty matrix"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Algorithm\n",
+ "\n",
+ "To get to row r and column c [r, c], we will need to have gone:\n",
+ "\n",
+ "* Right from [r, c-1] if this is a valid cell - [Path 1] \n",
+ "* Down from [r-1, c] if this is a valid cell - [Path 2]\n",
+ "\n",
+ "If we look at [Path 1], to get to [r, c-1], we will need to have gone:\n",
+ "\n",
+ "* Right from [r, c-2] if this is a valid cell\n",
+ "* Down from [r-1, c-1] if this is a valid cell\n",
+ "\n",
+ "Continue this process until we reach the start cell or until we find that there is no path.\n",
+ "\n",
+ "Base case:\n",
+ "\n",
+ "* If the input row or col are < 0, or if [row, col] is not a valid cell\n",
+ " * Return False\n",
+ "\n",
+ "Recursive case:\n",
+ "\n",
+ "We'll memoize the solution to improve performance.\n",
+ "\n",
+ "* Use the memo to see if we've already processed the current cell\n",
+ "* If any of the following is True, append the current cell to the path and set our result to True:\n",
+ " * We are at the start cell\n",
+ " * We get a True result from a recursive call on:\n",
+ " * [row, col-1]\n",
+ " * [row-1, col]\n",
+ "* Update the memo\n",
+ "* Return the result\n",
+ "\n",
+ "Complexity:\n",
+ "* Time: O(row * col)\n",
+ "* Space: O(row * col) for the recursion depth"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "metadata": {},
+ "source": [
+ "## Code"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 1,
+ "metadata": {
+ "collapsed": false
+ },
+ "outputs": [],
+ "source": [
+ "class Grid(object):\n",
+ "\n",
+ " def find_path(self, matrix):\n",
+ " if matrix is None or not matrix:\n",
+ " return None\n",
+ " cache = {}\n",
+ " path = []\n",
+ " if self._find_path(matrix, len(matrix) - 1, \n",
+ " len(matrix[0]) - 1, cache, path):\n",
+ " return path\n",
+ " else:\n",
+ " return None\n",
+ "\n",
+ " def _find_path(self, matrix, row, col, cache, path):\n",
+ " if row < 0 or col < 0 or not matrix[row][col]:\n",
+ " return False\n",
+ " cell = (row, col)\n",
+ " if cell in cache:\n",
+ " return cache[cell]\n",
+ " cache[cell] = (row == 0 and col == 0 or\n",
+ " self._find_path(matrix, row, col - 1, cache, path) or\n",
+ " self._find_path(matrix, row - 1, col, cache, path))\n",
+ " if cache[cell]:\n",
+ " path.append(cell)\n",
+ " return cache[cell]"
+ ]
+ },
+ {
+ "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_grid_path.py\n"
+ ]
+ }
+ ],
+ "source": [
+ "%%writefile test_grid_path.py\n",
+ "from nose.tools import assert_equal\n",
+ "\n",
+ "\n",
+ "class TestGridPath(object):\n",
+ "\n",
+ " def test_grid_path(self):\n",
+ " grid = Grid()\n",
+ " assert_equal(grid.find_path(None), None)\n",
+ " assert_equal(grid.find_path([[]]), None)\n",
+ " max_rows = 8\n",
+ " max_cols = 4\n",
+ " matrix = [[1] * max_cols for _ in range(max_rows)]\n",
+ " matrix[1][1] = 0\n",
+ " matrix[2][2] = 0\n",
+ " matrix[3][0] = 0\n",
+ " matrix[4][2] = 0\n",
+ " matrix[5][3] = 0\n",
+ " matrix[6][1] = 0\n",
+ " matrix[6][3] = 0\n",
+ " matrix[7][1] = 0\n",
+ " result = grid.find_path(matrix)\n",
+ " expected = [(0, 0), (1, 0), (2, 0),\n",
+ " (2, 1), (3, 1), (4, 1),\n",
+ " (5, 1), (5, 2), (6, 2), \n",
+ " (7, 2), (7, 3)]\n",
+ " assert_equal(result, expected)\n",
+ " matrix[7][2] = 0\n",
+ " result = grid.find_path(matrix)\n",
+ " assert_equal(result, None)\n",
+ " print('Success: test_grid_path')\n",
+ "\n",
+ "\n",
+ "def main():\n",
+ " test = TestGridPath()\n",
+ " test.test_grid_path()\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_grid_path\n"
+ ]
+ }
+ ],
+ "source": [
+ "%run -i test_grid_path.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/recursion_dynamic/grid_path/test_grid_path.py b/recursion_dynamic/grid_path/test_grid_path.py
new file mode 100644
index 0000000..9249cda
--- /dev/null
+++ b/recursion_dynamic/grid_path/test_grid_path.py
@@ -0,0 +1,39 @@
+from nose.tools import assert_equal
+
+
+class TestGridPath(object):
+
+ def test_grid_path(self):
+ grid = Grid()
+ assert_equal(grid.find_path(None), None)
+ assert_equal(grid.find_path([[]]), None)
+ max_rows = 8
+ max_cols = 4
+ matrix = [[1] * max_cols for _ in range(max_rows)]
+ matrix[1][1] = 0
+ matrix[2][2] = 0
+ matrix[3][0] = 0
+ matrix[4][2] = 0
+ matrix[5][3] = 0
+ matrix[6][1] = 0
+ matrix[6][3] = 0
+ matrix[7][1] = 0
+ result = grid.find_path(matrix)
+ expected = [(0, 0), (1, 0), (2, 0),
+ (2, 1), (3, 1), (4, 1),
+ (5, 1), (5, 2), (6, 2),
+ (7, 2), (7, 3)]
+ assert_equal(result, expected)
+ matrix[7][2] = 0
+ result = grid.find_path(matrix)
+ assert_equal(result, None)
+ print('Success: test_grid_path')
+
+
+def main():
+ test = TestGridPath()
+ test.test_grid_path()
+
+
+if __name__ == '__main__':
+ main()
\ No newline at end of file