岛屿的数量

 给你一个由 '1'（陆地）和 '0'（水）组成的的二维网格，请你计算网格中岛屿的数量。

岛屿总是被水包围，并且每座岛屿只能由水平方向和/或竖直方向上相邻的陆地连接形成。

此外，你可以假设该网格的四条边均被水包围。

示例 1：

输入：grid = [
  ["1","1","1","1","0"],
  ["1","1","0","1","0"],
  ["1","1","0","0","0"],
  ["0","0","0","0","0"]
]
输出：1

示例 2：

输入：grid = [
  ["1","1","0","0","0"],
  ["1","1","0","0","0"],
  ["0","0","1","0","0"],
  ["0","0","0","1","1"]
]
输出：3

示例代码：

class Solution:
    def numIslands(self, grid: List[List[str]]) -> int:
        row, col = len(grid), len(grid[0])
        ret = 0

        def dfs(x, y):
            grid[x][y] = '0'
            lst = [[1,0], [-1, 0], [0, 1], [0, -1]]
            for i in lst:
                nx, ny = x + i[0], y + i[1]
                if 0