island_perimeter.py

def island_perimeter(grid):
    perimeter = 0
    for i in range(len(grid)):
        row = grid[i]
        for j in range(len(row)):
            cell = grid[i][j]

            if cell != 1:
                continue

            top = grid[i - 1][j] if i != 0 else 0
            bottom = grid[i + 1][j] if i != len(grid) - 1 else 0
            left = grid[i][j - 1] if j != 0 else 0
            right = grid[i][j + 1] if j != len(row) - 1 else 0

            for other in [top, bottom, left, right]:
                if other == 0:
                    perimeter += 1
    return perimeter


grid1 = [
    [0, 0, 0, 0, 0, 0],
    [0, 1, 0, 0, 0, 0],
    [0, 1, 0, 0, 0, 0],
    [0, 1, 1, 1, 0, 0],
    [0, 0, 0, 0, 0, 0]
]
assert 12 == island_perimeter(grid1)

grid2 = [
    [1, 1, 1, 1, 1],
    [1, 1, 1, 1, 1],
    [1, 1, 1, 1, 1]
]
assert 16 == island_perimeter(grid2)

grid3 = [
    [0, 0, 0, 0, 0, 0],
    [0, 1, 1, 0, 0, 0],
    [1, 1, 1, 0, 0, 0],
    [0, 1, 1, 1, 0, 0],
    [0, 0, 0, 1, 1, 1]
]
assert 20 == island_perimeter(grid3)

grid4 = [
    [0, 1, 0, 0, 0, 1],
    [1, 1, 0, 0, 0, 1],
    [1, 1, 0, 1, 1, 1],
    [0, 1, 1, 1, 0, 0],
    [0, 0, 1, 1, 0, 0]
]
assert 28 == island_perimeter(grid4)