1def num_islands(grid):
2    rows, cols = len(grid), len(grid[0])
3    visited = set()
4    count = 0
5    def dfs(r, c):
6        if (r, c) in visited or not (0 <= r < rows and 0 <= c < cols) or grid[r][c] == 0:
7            return
8        visited.add((r, c))
9        for dr, dc in [(-1,0),(1,0),(0,-1),(0,1)]:
10            dfs(r + dr, c + dc)
11    for r in range(rows):
12        for c in range(cols):
13            if grid[r][c] == 1 and (r, c) not in visited:
14                dfs(r, c)
15                count += 1
16    return count