char[][] grid;

public int markIslands(int a, int b) {
    if (a < 0 || b < 0 || a >= grid.length || b >= grid[0].length || this.grid[a][b] == '0') {
        return 0;
    }
    this.grid[a][b] = '0';
    markIslands(a-1, b);
    markIslands(a+1, b);
    markIslands(a, b-1);
    markIslands(a, b+1);
    return 1;
}

public int numIslands(char[][] grid) {
    int count = 0;
    this.grid = grid;
    for (int r = 0; r < grid.length; r++) {
        for (int c = 0; c < grid[0].length; c++) {
            count += markIslands(r, c);
        }
    }
    return count;
}