200. 岛屿数量

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

提示：

• m == grid.length
• n == grid[i].length
• 1 <= m, n <= 300
• grid[i][j] 的值为 ‘0’ 或 ‘1’

Python 解答：

class Solution:
    def numIslands(self, grid: List[List[str]]) -> int:
        def dfs(grid, x, y):
            if grid[x][y] == '1':
                grid[x][y] = '2'
            else:
                return 
            if x+1 < len(grid):
                dfs(grid, x+1, y)
            if x-1 > -1:
                dfs(grid, x-1, y)
            if y+1 < len(grid[0]):
                dfs(grid, x, y+1)
            if y-1 > -1:
                dfs(grid, x, y-1)

        total = 0
        for i in range(len(grid)):
            for j in range(len(grid[0])):
                if grid[i][j] == '1':
                    dfs(grid, i, j)
                    total += 1
        return total