Swift でクイックソート

extension Array where Element == Int {
    func qsort() -> [Int] {
        if self.isEmpty {return []}
        var xs = self
        let pivot = xs.removeFirst()
        let left  = xs.filter({$0 < pivot})
        let right = xs.filter({$0 > pivot})
        return left.qsort() + Array(arrayLiteral: pivot) + right.qsort()
    }
}

print([5, 1, 3, 9, 8, 7].qsort())    //=>[1, 3, 5, 7, 8, 9]

Swift 3.1-dev.
　
※参考
Swift 練習 - Marginalia