QuickSort.swift

import Foundation

extension Array where Element: Comparable {
  func quickSort() -> [Element] {
    guard let first = self.first else { return [] }
    let tail = self.dropFirst()
   
    let lessPart = tail.filter { $0 <= first }.quickSort()
    let morePart = tail.filter { $0 > first }.quickSort()
    
    let result = lessPart + [first] + morePart
    
    return result
  }
}

[7,5,4,4,5,1,8,87,5,4].quickSort() // [1, 4, 4, 4, 5, 5, 5, 7, 8, 87]