My Java Solution:

    int rowSize = arr.size();
    int leftSum = IntStream.range(0, rowSize)
                           .map(i -> arr.get(i).get(i))
                           .sum();
    int rightSum = IntStream.range(0, rowSize)
                            .map(i -> arr.get(i).get(rowSize - (1 + i)))
                            .sum();
    return Math.abs(leftSum - rightSum);