Added Quicksort

* Added selection sort

* Updated SelectionSort according to coding standards.

* Changed the incorrect function call in testSelectionSort3.

* Added Quicksort.

* added description of quick sort.

Author: bhavyakaria

src/main/kotlin/sort/QuickSort.kt

@@ -0,0 +1,53 @@
+package sort
+
+/**
+ * This method implements the Quick Sort
+ *
+ * @param array The array to be sorted
+ * It is a Divide and Conquer algorithm. It picks an element as pivot and partitions the given array around the picked pivot.
+ *
+ * Worst-case performance	    O(n^2)
+ * Best-case performance	    O(nLogn)
+ * Average performance      	O(nLogn)
+ * Worst-case space complexity	O(1)
+ **/
+fun <T: Comparable<T>> quickSort(array: Array<T>, low: Int, high: Int) {
+    if (low < high) {
+        val pivot = partition(array, low, high)
+        quickSort(array, low, pivot - 1)
+        quickSort(array, pivot, high)
+    }
+}
+
+/**
+ * This method finds the pivot index for an array
+ *
+ * @param array The array to be sorted
+ * @param low The first index of the array
+ * @param high The last index of the array
+ *
+ * */
+fun <T: Comparable<T>> partition(array: Array<T>, low: Int, high: Int): Int {
+
+    var left = low
+    var right = high
+    val mid = (left + right) / 2
+    val pivot = array[mid]
+
+    while (left <= right) {
+        while (array[left] < pivot) {
+            left++
+        }
+
+        while (array[right] > pivot) {
+            right--
+        }
+
+        if (left <= right) {
+            swapElements(array, left, right)
+            left++
+            right--
+        }
+    }
+    return left
+}

src/test/kotlin/sort/QuickSortTest.kt

@@ -0,0 +1,31 @@
+package sort
+
+import org.junit.Assert.assertArrayEquals
+import org.junit.Test
+
+class QuickSortTest {
+
+    @Test
+    fun testQuickSort1() {
+        val array = arrayOf(4, 3, 2, 8, 1)
+        quickSort(array, 0, array.size - 1)
+
+        assertArrayEquals(array, arrayOf(1, 2, 3, 4, 8))
+    }
+
+    @Test
+    fun testQuickSort2() {
+        val array = arrayOf("A", "D", "E", "C", "B")
+        quickSort(array, 0, array.size - 1)
+
+        assertArrayEquals(array, arrayOf("A", "B", "C", "D", "E"))
+    }
+
+    @Test
+    fun testQuickSort3() {
+        val array = arrayOf(20, 5, 16, -1, 6)
+        quickSort(array, 0, array.size - 1)
+
+        assertArrayEquals(array, arrayOf(-1, 5, 6, 16, 20))
+    }
+}