See http://ericasadun.com/2016/06/28/make-it-swifter-challenge/

selection.swift

func selectionSort(_ array: [Int]) -> [Int] {
    guard let minValue = array.min(), let index = array.index(of: minValue) else {
        return []
    }

    let ranges = [0..<index, index.advanced(by: 1)..<array.endIndex]
    return [minValue] + selectionSort(ranges.lazy.flatMap { array[$0] })
}