Collection+TransposeTwoDimensionalArray.swift

extension Collection where Element: Collection {
    func transposed() -> [[Element.Element]]? {
        guard let maxCount = self.map({$0.count}).max() else {return nil}
        var result = [[Element.Element?]](repeating: [Element.Element?](repeating: nil, count: self.count), count: maxCount)
        for (r,row) in self.enumerated() {
            for (c,v) in row.enumerated() {
                result[c][r] = v
            }
        }
        return result
            .map { (elements) -> [Element.Element] in
                elements.compactMap{$0}
        }
    }
}

let test1 = [
    [],
    [1,2,3],
    [4,5]
]
let test2 = [
    [1,2],
    [3,4,5,6,7],
    [],
    [8],
    []
]

test1.transposed() == [[1,4], [2,5], [3]]
test2.transposed() == [[1,3,8], [2,4], [5], [6], [7]]