fun diagonalDifference(arr: Array<Array<Int>>): Int {
    val sumLtoR = arr.foldIndexed(0) { index, accumulator, element ->
        accumulator + element[index]
    }
    val sumRtoL = arr.foldIndexed(0) { index, accumulator, element ->
        accumulator + element[element.lastIndex - index]
    }
    return abs(sumLtoR - sumRtoL)
}