Add doctest on quickselect algorithm

Author: mhihasan

Data-Structures/Array/QuickSelect.js

@@ -3,15 +3,19 @@
  *
  * Notes:
  * -QuickSelect is related to QuickSort, thus has optimal best and average
- *  case (O(n)) but unlikely poor worst case (O(n^2))
+ * -case (O(n)) but unlikely poor worst case (O(n^2))
  * -This implementation uses randomly selected pivots for better performance
  *
  * @complexity: O(n) (on average )
  * @complexity: O(n^2) (worst case)
  * @flow
  */
 
-function QuickSelect (items, kth) {
+function QuickSelect (items, kth) { // eslint-disable-line no-unused-vars
+  if (kth < 1 || kth > items.length) {
+    return 'Index Out of Bound'
+  }
+
   return RandomizedSelect(items, 0, items.length - 1, kth)
 }
 
@@ -62,5 +66,13 @@ function Swap (arr, x, y) {
   [arr[x], arr[y]] = [arr[y], arr[x]]
 }
 
-// testing
-console.log(QuickSelect([1, 4, 2, -2, 4, 5]))
+// > QuickSelect([1, 4, 2, -2, 4, 5], 1)
+// -2
+// > QuickSelect([1, 4, 2, -2, 4, 5], 5)
+// 4
+// > QuickSelect([1, 4, 2, -2, 4, 5], 6)
+// 5
+// > QuickSelect([1, 4, 2, -2, 4, 5], -1)
+// "Index Out of Bound"
+// > QuickSelect([1, 4, 2, -2, 4, 5], 7)
+// "Index Out of Bound"