200. Number of Islands - LeetCode

class Solution:
    def numIslands(self, grid) -> int:
        M, N = len(grid), len(grid[0])

        def dfs(m, n):
            r = 0 <= m < M and 0 <= n < N and grid[m][n] == '1'
            if r:
                grid[m][n] = None
                list(map(dfs, (m + 1, m - 1, m, m), (n, n, n + 1, n - 1)))
            return r

        return sum(dfs(m, n) for m in range(M) for n in range(N))