Graph Traversals

def count_islands(grid):

    if not grid:

        return 0

    rows, cols = len(grid), len(grid[0])

    islands = 0

    def dfs(r, c):

        if r < 0 or r >= rows or c < 0 or c >= cols or grid[r][c] != 1:

            return

        grid[r][c] = 0  # mark visited

        dfs(r + 1, c)

        dfs(r - 1, c)

        dfs(r, c + 1)

        dfs(r, c - 1)

    for r in range(rows):

        for c in range(cols):

            if grid[r][c] == 1:

                islands += 1

                dfs(r, c)

    return islands