How to get the summation of diagonal lines using higher-order functions?

Question

Consider the following 2D array:

let array = [
                [11, 2, 4],
                [4, 5, 6],
                [10, 8, -12]
            ]

Answer 1

To get the first sum, you want the i'th element of the i'th row:

let firstDiag = array.enumerated().map { $1[$0] }.reduce(0, +)

To get the second sum, you want the same thing, but the columns reversed:

let secondDiag = array.enumerated().map { $1.reversed()[$0] }.reduce(0, +)