// looping through the grid
    for(int i = 0; i < grid.length; i++) {
        for(int j = 0; j < grid[0].length; j++) {
            
            // if not visited & it's an island
            if(visited[i][j] != 1 && grid[i][j] == 1) {
                int area = calcArea(grid, i, j, visited);
                max = Math.max(max, area);
            }
        }
    }
    
    return max;
}

private int calcArea(int[][] grid, int row, int col, int[][] visited) {
    
    // out of bound
    if(row < 0 || row >= grid.length || col < 0 || col >= grid[0].length) {
        return 0;
    }
    
    // visited before, don't count it again
    if(visited[row][col] == 1) {
        return 0;
    } else {
        // mark as visted
        visited[row][col] = 1;
    }
    
    // water
    if(grid[row][col] == 0) {
        return 0;
    }
    
    
    // check up, down, bottom, right
    return 1 + calcArea(grid, row - 1, col, visited)  + 
        calcArea(grid, row + 1, col, visited)
        + calcArea(grid, row, col - 1, visited) + calcArea(grid, row, col + 1, visited);
}