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'

深度优先遍历，每次dfs就是把当前点变成0

var numIslands = function(grid) {
    function dfs(x,y){
        grid[x][y] = 0;
        if (grid[x - 1] && grid[x - 1][y] == 1) dfs(x - 1, y);
        if (grid[x + 1] && grid[x + 1][y] == 1) dfs(x + 1, y);
        if (grid[x][y - 1] == 1) dfs(x, y - 1);
        if (grid[x][y + 1] == 1) dfs(x, y + 1);
    }

    let count = 0;
    for (let i = 0; i < grid.length; i++){
        for (let j = 0; j < grid[0].length; j++){
            if (grid[i][j] == 1) {
                dfs(i, j);
                count++;
            }
        }
    }
    return count;
};