Number of islands

In a grid of land (1) and water (0), count the islands. Land touching up, down, left or right is the same island.

Press Play or Step to walk through the example, or change the input and press Run. Each frame says what the algorithm does and why.

def num_islands(grid):
    R, C, count = len(grid), len(grid[0]), 0
    def fill(r, c):
        if not (0 <= r < R and 0 <= c < C) or grid[r][c] != '1': return
        grid[r][c] = '0'                      # mark visited
        fill(r+1, c); fill(r-1, c); fill(r, c+1); fill(r, c-1)
    for r in range(R):
        for c in range(C):
            if grid[r][c] == '1':
                count += 1; fill(r, c)
    return count