Add an initial implementation for the QuickSort algorithm

sorts/QuickSort.swift

@@ -0,0 +1,70 @@
+import Foundation
+
+extension Array where Element: Comparable {
+    /// Sorts the array using the QuickSort algorithm in place.
+    ///
+    /// The QuickSort algorithm sorts the array by first choosing a pivot. This pivot is used to rearrange
+    /// all elements, moving the smaller ones to the left of it. This operation is then recursevely applied
+    /// to the subarrays formed around the pivot.
+    mutating func quickSort() {
+        guard count > 1 else {
+            return
+        }
+
+        _quickSort(from: 0, to: count - 1)
+    }
+
+    mutating private func _quickSort(from left: Int, to right: Int) {
+        guard left < right, right - left > 0 else {
+            return
+        }
+
+        let pivotIndex = partition(from: left, to: right)
+        _quickSort(from: left, to: pivotIndex - 1)
+        _quickSort(from: pivotIndex + 1, to: right)
+    }
+
+    /// This method is where the pivot is chosen, so the smaller elements get moved to the left,
+    /// and the bigger ones to the right.
+    mutating private func partition(from left: Int, to right: Int) -> Int {
+        /// Chooses the pivot, which in this case is always the first element, which is not very efficient.
+        let pivotIndex = left
+        swapAt(pivotIndex, right)
+
+        let pivot = self[right]
+        var i = left
+
+        for j in i ..< right {
+            // If the element is smaller than the pivot, move it to the left.
+            if self[j] <= pivot {
+                swapAt(i, j)
+                i += 1
+            }
+        }
+
+        // Move the pivot to its right sorted position.
+        swapAt(i, right)
+
+        return i
+    }
+
+    /// Returns a sorted version of this array using the QuickSort algorithm.
+    func quickSorted() -> Self {
+        var copy = self
+
+        copy.quickSort()
+
+        return copy
+    }
+}
+
+// Use the following code to test it:
+// var numbers = [1002, 42, 55, 124, 205]
+// debugPrint(numbers.quickSorted())
+//
+// numbers.quickSort()
+// debugPrint(numbers)
+//
+// The console should print:
+// [42, 55, 124, 205, 1002]
+// [42, 55, 124, 205, 1002]