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。

    来源：力扣（LeetCode）
    链接：https://leetcode-cn.com/problems/number-of-islands
    著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

    示例 1：

    1. 输入：grid = [
    2.   ["1","1","1","1","0"],
    3.   ["1","1","0","1","0"],
    4.   ["1","1","0","0","0"],
    5.   ["0","0","0","0","0"]
    6. ]
    7. 输出：1
    9. 来源：力扣（LeetCode）
    10. 链接：https://leetcode-cn.com/problems/number-of-islands
    11. 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

    示例 2：

    1. 输入：grid = [
    2.   ["1","1","0","0","0"],
    3.   ["1","1","0","0","0"],
    4.   ["0","0","1","0","0"],
    5.   ["0","0","0","1","1"]
    6. ]
    7. 输出：3
    9. 来源：力扣（LeetCode）
    10. 链接：https://leetcode-cn.com/problems/number-of-islands
    11. 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

    答案

    1. function numIslands (grid) {
    2.   let num =0;
    3.   function overturn(i, j) {
    4.       if (i<0||j<0||i>grid.length-1||j>grid[0].length-1) return
    5.     if (grid[i][j] === "1") {
    6.         grid[i][j] = "0"
    7.       overturn(i, j+1)
    8.       overturn(i, j-1)
    9.       overturn(i+1, j)
    10.       overturn(i-1, j)
    11.     }
    12.   }
    13.   if (grid & grid.length) {
    14.           for (let i =0; i<grid.length;i++) {
    15.           for (let j =0; j< grid.length; j++) {
    16.             if (grid[i][j] === "1") {
    17.               num ++
    18.             overturn(i,j)
    19.           }
    20.         }
    21.       }
    22.   }
    24. }