quick sort and findKthElement in array

scala/src/main/scala/ch12_sorts/QuickSort.scala

@@ -0,0 +1,54 @@
+package ch12_sorts
+
+object QuickSort {
+
+  //find the K th smallest element int the array
+  def findKthElement(items: Array[Int], k: Int): Int = {
+    _findKthElement(items, k, 0, items.length - 1)
+  }
+
+  private[this] def _findKthElement(items: Array[Int], k: Int, p: Int, r: Int): Int = {
+    val q = _partition(items, p, r)
+
+    if (k == q + 1) {
+      items(q)
+    } else if (k < q + 1) {
+      _findKthElement(items, k, p, q - 1)
+    } else {
+      _findKthElement(items, k, q + 1, r)
+    }
+  }
+
+  def quickSort(items: Array[Int]): Array[Int] = {
+    _quickSort(items, 0, items.length - 1)
+    items
+  }
+
+  private[this] def _quickSort(items: Array[Int], p: Int, r: Int): Unit = {
+    if (p >= r) {
+      return
+    }
+    val q = _partition(items, p, r)
+    _quickSort(items, p, q - 1)
+    _quickSort(items, q + 1, r)
+  }
+
+  private[this] def _partition(items: Array[Int], p: Int, r: Int): Int = {
+    val pivot = items(r)
+    var i = p
+    for (j <- Range(p, r)) {
+      if (items(j) < pivot) {
+        val temp = items(i)
+        items(i) = items(j)
+        items(j) = temp
+        i += 1
+      }
+    }
+
+    val temp = items(i)
+    items(i) = items(r)
+    items(r) = temp
+
+    i
+  }
+}

scala/src/test/scala/ch11_sorts/SortsTest.scala

@@ -1,5 +1,6 @@
 package ch11_sorts
 
+import ch12_sorts.{MergeSort, QuickSort}
 import org.scalatest.{FlatSpec, Matchers}
 
 import scala.util.Random
@@ -50,6 +51,8 @@ class SortsTest extends FlatSpec with Matchers {
     timed("bubbleSort", Sorts.bubbleSort, array.clone())
     timed("insertSort", Sorts.insertSort, array.clone())
     timed("selectionSort", Sorts.selectionSort, array.clone())
+    timed("mergeSort", MergeSort.mergeSort, array.clone())
+    timed("quickSort", QuickSort.quickSort, array.clone())
   }
 
   def reportElapsed(name: String, time: Long): Unit = println(name + " takes in " + time + "ms")

scala/src/test/scala/ch12_sorts/QuickSortTest.scala

@@ -0,0 +1,31 @@
+package ch12_sorts
+
+import org.scalatest.{FlatSpec, Matchers}
+
+class QuickSortTest extends FlatSpec with Matchers {
+
+  behavior of "QuickSortTest"
+
+  it should "quickSort" in {
+    var array = Array(4, 5, 6, 3, 2, 1)
+    array = QuickSort.quickSort(array)
+    array.mkString("") should equal("123456")
+
+    array = Array(4)
+    array = QuickSort.quickSort(array)
+    array.mkString("") should equal("4")
+
+    array = Array(4, 2)
+    array = QuickSort.quickSort(array)
+    array.mkString("") should equal("24")
+  }
+
+  it should "find the Kth element in the array" in {
+    val array = Array(4, 2, 5, 12, 3)
+
+    QuickSort.findKthElement(array, 3) should equal(4)
+    QuickSort.findKthElement(array, 5) should equal(12)
+    QuickSort.findKthElement(array, 1) should equal(2)
+  }
+
+}