public int numIslands(char[][] grid) {

int n = grid.length;

if(n == 0) return 0;

int m = grid[0].length;

if(m == 0) return 0;

int count = 0;

for(int i=0; i < n; i++){

for(int j=0; j < m; j++){

if(grid[i][j] == '1'){

count++;

dfsMark(grid, i, j);

}

}

}

return count;

}

public void dfsMark(char[][] grid, int i, int j){

if(i < 0 || j < 0 || i > grid.length-1 || j > grid[0].length-1 || grid[i][j] != '1'){

return;

}

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

dfsMark(grid, i+1, j);

dfsMark(grid, i-1, j);

dfsMark(grid, i, j+1);

dfsMark(grid, i, j-1);

}

}