This notebook was prepared by Donne Martin. Source and license info is on GitHub.

# Solution Notebook¶

## Constraints¶

• Are there restrictions to how the robot moves?
• The robot can only move right and down
• Are some cells invalid (off limits)?
• Yes
• Can we assume the starting and ending cells are valid cells?
• Yes
• Is this a rectangular grid? i.e. the grid is not jagged?
• Yes
• Will there always be a valid way for the robot to get to the bottom right?
• No, return None
• Can we assume the inputs are valid?
• No
• Can we assume this fits memory?
• Yes

## Test Cases¶

o = valid cell
x = invalid cell

0  1  2  3
0  o  o  o  o
1  o  x  o  o
2  o  o  x  o
3  x  o  o  o
4  o  o  x  o
5  o  o  o  x
6  o  x  o  x
7  o  x  o  o

• General case
expected = [(0, 0), (1, 0), (2, 0),
(2, 1), (3, 1), (4, 1),
(5, 1), (5, 2), (6, 2),
(7, 2), (7, 3)]
• No valid path, say row 7, col 2 is invalid
• None input
• Empty matrix

## Algorithm¶

To get to row r and column c [r, c], we will need to have gone:

• Right from [r, c-1] if this is a valid cell - [Path 1]
• Down from [r-1, c] if this is a valid cell - [Path 2]

If we look at [Path 1], to get to [r, c-1], we will need to have gone:

• Right from [r, c-2] if this is a valid cell
• Down from [r-1, c-1] if this is a valid cell

Continue this process until we reach the start cell or until we find that there is no path.

Base case:

• If the input row or col are < 0, or if [row, col] is not a valid cell
• Return False

Recursive case:

We'll memoize the solution to improve performance.

• Use the memo to see if we've already processed the current cell
• If any of the following is True, append the current cell to the path and set our result to True:
• We are at the start cell
• We get a True result from a recursive call on:
• [row, col-1]
• [row-1, col]
• Update the memo
• Return the result

Complexity:

• Time: O(row * col)
• Space: O(row * col) for the recursion depth

## Code¶

In :
class Grid(object):

def find_path(self, matrix):
if matrix is None or not matrix:
return None
cache = {}
path = []
if self._find_path(matrix, len(matrix) - 1,
len(matrix) - 1, cache, path):
return path
else:
return None

def _find_path(self, matrix, row, col, cache, path):
if row < 0 or col < 0 or not matrix[row][col]:
return False
cell = (row, col)
if cell in cache:
return cache[cell]
cache[cell] = (row == 0 and col == 0 or
self._find_path(matrix, row, col - 1, cache, path) or
self._find_path(matrix, row - 1, col, cache, path))
if cache[cell]:
path.append(cell)
return cache[cell]


## Unit Test¶

In :
%%writefile test_grid_path.py
import unittest

class TestGridPath(unittest.TestCase):

def test_grid_path(self):
grid = Grid()
self.assertEqual(grid.find_path(None), None)
self.assertEqual(grid.find_path([[]]), None)
max_rows = 8
max_cols = 4
matrix = [ * max_cols for _ in range(max_rows)]
matrix = 0
matrix = 0
matrix = 0
matrix = 0
matrix = 0
matrix = 0
matrix = 0
matrix = 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)]
self.assertEqual(result, expected)
matrix = 0
result = grid.find_path(matrix)
self.assertEqual(result, None)
print('Success: test_grid_path')

def main():
test = TestGridPath()
test.test_grid_path()

if __name__ == '__main__':
main()

Overwriting test_grid_path.py

In :
%run -i test_grid_path.py

Success: test_grid_path