In 2D binary matrix find the number of islands

Question

I am trying to count the number of islands (a group of connected 1s forms an island) in a 2D binary matrix.

Example:

[
[1, 1, 0, 0, 0],
[0, 1, 0, 0,          


        

Answer 1

Your algorithm is almost correct except for the line 21:

if r != i and c != j:
    cnt += count_houses(mat, visited, r, c)

Instead you want to use or as you want to continue counting provided at least one of the coordinate is not the same as your center.

if r != i or c != j:
    cnt += count_houses(mat, visited, r, c)

An alternate and more intuitive way to write this would be the following

if (r, c) != (i, j):
    cnt += count_houses(mat, visited, r, c)

Answer 2

big hammer approach, for reference

had to add structure argument np.ones((3,3)) to add diagonal connectivity

import numpy as np
from scipy import ndimage

ary = np.array([
[1, 1, 0, 0, 0],
[0, 1, 0, 0, 1],
[1, 0, 0, 1, 1],
[0, 0, 0, 0, 0],
[1, 0, 1, 0, 1]
])

labeled_array, num_features = ndimage.label(ary, np.ones((3,3)))

labeled_array, num_features
Out[183]: 
(array([[1, 1, 0, 0, 0],
        [0, 1, 0, 0, 2],
        [1, 0, 0, 2, 2],
        [0, 0, 0, 0, 0],
        [3, 0, 4, 0, 5]]), 5)