var sumLeftToRight: Int = 0
var sumRightToLeft: Int = 0
var arr: [[Int]] = []

for i in 0..<arr.count {
   sumLeftToRight += arr[i][i]
   sumRightToLeft += arr[i][arr.count-i-1]
}

//Swift 5 Solution within a function with a bit different approach in case negative numbers in test-cases 
func sumDiagonally(arr:[[Int]])-> Int{
    var subn = 0
    var sum1st = 0
    var sum2nd = 0
    for i in 0..<arr.count{
        sum1st += arr[i][i]
        sum2nd += arr[i][arr.count-i-1]
        subn = sum1st - sum2nd
            if sum1st < sum2nd{
            subn = subn * -1
            }
        }
    return subn
}
sumDiagonally(arr: [[1,2,3],
                    [4,5,6],
                    [9,8,9]])