对数组index的判断要放前面，要不然报数组越界异常。

import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 判断岛屿数量
     * @param grid char字符型二维数组 
     * @return int整型
     */
    public void dfs(char[][] grid, int i, int j){
        int n = grid.length;
        int m = grid[0].length;

        grid[i][j] = '0';

        if(i - 1 >= 0 && grid[i-1][j] == '1')//上
            dfs(grid, i-1, j);
        if(i + 1 < n && grid[i+1][j] == '1')//下
            dfs(grid, i+1, j);
        if(j - 1 >= 0 && grid[i][j-1] == '1')//左
            dfs(grid, i, j-1);
        if(j + 1 < m && grid[i][j+1] == '1')//右
            dfs(grid, i, j+1);

    }


    public int solve (char[][] grid) {
        // write code here
        int n = grid.length;
        if(n == 0) return 0;
        int m = grid[0].length;

        int count = 0;
        for(int i = 0; i < n; i++){
            for(int j = 0; j < m; j++){
                if(grid[i][j] == '1'){
                    count++;
                    dfs(grid, i , j);
                }
            }
        }

        return count;
    }
}