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

思路：遍历网格，当前为1就是一个岛屿，然后以当前为中心，向左右上下扩散，把相邻的1置为0。

代码：

/*** @param {character[][]} grid* @return {number}*/
var numIslands = function (grid) {let res = 0let zero = (y, x) => { // 置0let left = x - 1, right = x + 1, top = y - 1, bottom = y + 1while (grid[y][left] === '1') { // 左扩散grid[y][left] = '0'zero(y, left)left--}while (grid[y][right] === '1') { // 右扩散grid[y][right] = '0'zero(y, right)right++}while (top >= 0 && grid[top][x] === '1') { // 上扩散grid[top][x] = '0'zero(top, x)top--}while (bottom < grid.length && grid[bottom][x] === '1') { // 下扩散grid[bottom][x] = '0'zero(bottom, x)bottom++}}for (let y = 0; y < grid.length; y++) { // 找1for (let x = 0; x < grid[0].length; x++) {if (grid[y][x] === '1') {res++zero(y, x)}}}return res
};