add quick sort in swift

swift/12_sorts/QuickSort.swift

@@ -0,0 +1,36 @@
+//
+//  QuickSort.swift
+//  algo
+//
+//  Created by Wenru Dong on 2018/10/17.
+//  Copyright © 2018年 Wenru Dong. All rights reserved.
+//
+
+import Foundation
+
+public func quickSort<T: RandomAccessCollection & MutableCollection>(_ a: inout T) where T.Element: Comparable {
+    quickSort(&a, from: a.startIndex, to: a.index(before: a.endIndex))
+}
+
+fileprivate func quickSort<T: RandomAccessCollection & MutableCollection>(_ a: inout T, from low: T.Index, to high: T.Index) where T.Element: Comparable {
+    if low >= high { return }
+
+    let m = partition(&a, from: low, to: high)
+    quickSort(&a, from: low, to: a.index(before: m))
+    quickSort(&a, from: a.index(after: m), to: high)
+}
+
+fileprivate func partition<T: RandomAccessCollection & MutableCollection>(_ a: inout T, from low: T.Index, to high: T.Index) -> T.Index where T.Element: Comparable {
+    let pivot = a[low]
+    var j = low
+    var i = a.index(after: low)
+    while i <= high {
+        if a[i] < pivot {
+            a.formIndex(after: &j)
+            a.swapAt(i, j)
+        }
+        a.formIndex(after: &i)
+    }
+    a.swapAt(low, j)
+    return j
+}

swift/12_sorts/SortsTests.swift

@@ -34,6 +34,20 @@ class SortsTests: XCTestCase {
         XCTAssertEqual(a3, [-2, -1, 0, 3, 3, 6, 7, 8, 9, 9])
     }
 
+    func testQuickSort() {
+        var a1 = [1, 1, 1, 1]
+        quickSort(&a1)
+        XCTAssertEqual(a1, [1, 1, 1, 1])
+
+        var a2 = [4, 3, 2, 1]
+        quickSort(&a2)
+        XCTAssertEqual(a2, [1, 2, 3, 4])
+
+        var a3 = [3, 6, 9, 7, 8, -1, 9, 3, -2, 0]
+        quickSort(&a3)
+        XCTAssertEqual(a3, [-2, -1, 0, 3, 3, 6, 7, 8, 9, 9])
+    }
+
     func testPerformanceExample() {
         // This is an example of a performance test case.
         self.measure {