Added Merge sort

* Added selection sort

* Updated SelectionSort according to coding standards.

* Changed the incorrect function call in testSelectionSort3.

* Added Quicksort.

* added description of quick sort.

* Added Merge sort.

Author: bhavyakaria

src/main/kotlin/sort/MergeSort.kt

@@ -0,0 +1,57 @@
+package sort
+
+/**
+ * This function implements Merge Sort
+ *
+ * @param array The array to be sorted
+ * It is a Divide and Conquer algorithm. It sorts the array by dividing it into two halves and comparing the elements.
+ *
+ * Worst-case performance	    O(n log n)
+ * Best-case performance	    O(n log n)
+ * Average performance      	O(n log n)
+ * Worst-case space complexity	O(n)
+ */
+fun <T: Comparable<T>> mergeSort(array: Array<T>, start: Int, end: Int) {
+
+    val temp = arrayOfNulls<Comparable<*>>(array.size) as Array<T>
+
+    if (start < end) {
+        val mid = start + (end - start) / 2
+        mergeSort(array, start, mid)
+        mergeSort(array, mid + 1, end)
+        merge(array, temp, start, mid, end)
+    }
+}
+
+/**
+ * This function merges the two halves after comparing them
+ * @param array The array to be sorted
+ * @param temp The temp array containing the values
+ * @param start Starting index of the array
+ * @param mid Middle index of the array
+ * @param end Ending index of the array
+ */
+fun <T: Comparable<T>> merge(array: Array<T>, temp: Array<T>, start: Int, mid: Int, end: Int) {
+
+    System.arraycopy(array, start, temp, start, end - start + 1)
+
+    var i = start
+    var j = mid + 1
+    var k = start
+
+    while (i <= mid && j <= end) {
+        if (temp[i] < temp[j]) {
+            array[k++] = temp[i++]
+        } else {
+            array[k++] = temp[j++]
+        }
+    }
+
+    while (i <= mid) {
+        array[k++] = temp[i++]
+    }
+
+    while (j <= end) {
+        array[k++] = temp[j++]
+    }
+}

src/test/kotlin/sort/MergeSortTest.kt

@@ -0,0 +1,31 @@
+package sort
+
+import org.junit.Assert
+import org.junit.Test
+
+class MergeSortTest {
+
+    @Test
+    fun testMergeSort1() {
+        val array = arrayOf(4, 3, 2, 8, 1)
+        mergeSort(array, 0, array.size - 1)
+
+        Assert.assertArrayEquals(array, arrayOf(1, 2, 3, 4, 8))
+    }
+
+    @Test
+    fun testMergeSort2() {
+        val array = arrayOf("A", "D", "E", "C", "B")
+        mergeSort(array, 0, array.size - 1)
+
+        Assert.assertArrayEquals(array, arrayOf("A", "B", "C", "D", "E"))
+    }
+
+    @Test
+    fun testMergeSort3() {
+        val array = arrayOf(20, 5, 16, -1)
+        mergeSort(array, 0, array.size - 1)
+
+        Assert.assertArrayEquals(array, arrayOf(-1, 5, 16, 20))
+    }
+}