Quick Sort implementation.

Algorithms/Sorting/QuickSort.playground/Contents.swift

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+/// Quick Sort Algorithm.
+/// Didive and conquer.
+/// 1. Pick a pivot point, partition the array.
+/// 2. Values to left of pivot are smaller, values to right are larger
+/// 3. Repeat steps 1 & 2 and sub-arrays
+
+public func quickSort(array: inout [Int]) {
+    quickSort(array: &array, lowIndex: 0, highIndex: array.count - 1)
+}
+
+private func quickSort(array: inout [Int], lowIndex: Int, highIndex: Int) {
+    if lowIndex < highIndex {
+        let partitionIndex = partition(array: &array, lowIndex: lowIndex, highIndex: highIndex)
+        quickSort(array: &array, lowIndex: lowIndex, highIndex: partitionIndex - 1)
+        quickSort(array: &array, lowIndex: partitionIndex + 1, highIndex: highIndex)
+    }
+}
+
+private func partition(array: inout [Int], lowIndex: Int, highIndex: Int) -> Int {
+    // Select last element as pivot value
+    let pivotValue = array[highIndex]
+
+    // Lowest element index
+    var lowestIndex = lowIndex
+    var temp: Int
+
+    // Iterate from low to high, but exclude pivot
+    for index in lowIndex..<highIndex {
+        if array[index] <= pivotValue {
+            // Swap current element with lower element
+            temp = array[lowestIndex]
+            array[lowestIndex] = array[index]
+            array[index] = temp
+
+            // Increment lowest index
+            lowestIndex += 1
+        }
+    }
+
+    // Move pivot into new position where lowest index left off
+    temp = array[lowestIndex]
+    array[lowestIndex] = array[highIndex]
+    array[highIndex] = temp
+
+    // Return calculated partition (pivot) index
+    return lowestIndex
+}
+
+var unsortedArray = [8, 6, 4, 100, 67, 1, 98, 7]
+var copy = unsortedArray
+copy.sort()
+quickSort(array: &unsortedArray)
+print(unsortedArray)
+print(copy)
+assert(copy == unsortedArray)

Algorithms/Sorting/QuickSort.playground/contents.xcplayground

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+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<playground version='5.0' target-platform='ios'>
+    <timeline fileName='timeline.xctimeline'/>
+</playground>