public static int hourglassSum(List<List<Integer>> arr) {
        if(arr.size() < 3) return 0; //no possible hourglass
        
        int max = Integer.MIN_VALUE;
        
        //iterate over each row except the last two
        int i = 0;
        while(i < arr.size() - 2){
            //iterate over each col except the first & last
            int j = 1;
            while(j < arr.get(i).size() - 1){
                int currMax = calcHourglassSum(arr, i, j); //get the curr max
                max = Math.max(currMax, max); //update the max
                j++;
            }
            i++;
        }
        return max;
    }
    
    public static int calcHourglassSum(List<List<Integer>> arr, int i, int j){
        return 
            arr.get(i).get(j) + arr.get(i).get(j-1) + arr.get(i).get(j+1)
            + arr.get(i+1).get(j) + arr.get(i+2).get(j) + arr.get(i+2).get(j-1) 
            + arr.get(i+2).get(j+1);
    }