1. 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。
    3. 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。
    5. 此外，你可以假设该网格的四条边均被水包围。
    7. 示例 1：
    8. 输入：grid = [
    9.   ["1","1","1","1","0"],
    10.   ["1","1","0","1","0"],
    11.   ["1","1","0","0","0"],
    12.   ["0","0","0","0","0"]
    13. ]
    14. 输出：1
    1. class Solution {
    2.     public int numIslands(char[][] grid) {
    3.         if(grid == null && grid.length == 0) return 0;
    4.         int cnt = 0;
    5.         for (int x = 0; x < grid.length; x++) {
    6.             for (int y = 0; y < grid[0].length; y++) {
    7.                 if (grid[x][y] == '1') {
    8.                     cnt++;
    9.                     dfs(grid, x, y);
    10.                 }
    11.             }
    12.         }
    13.          return cnt;
    14.     }
    16.     public void dfs(char[][] grid,int x,int y){
    18.         if(x >= grid.length || x < 0 || y >= grid[0].length || y < 0 
    19.            || grid[x][y] == '0') return;
    21.         grid[x][y] = '0';
    22.         dfs(grid,x+1,y);
    23.         dfs(grid,x-1,y);
    24.         dfs(grid,x,y+1);
    25.         dfs(grid,x,y-1);
    26.     }
    27. }