Re-orgainze files and folders in the repository (#172)

* Re-orgainze files and folders in the repository

* updating DIRECTORY.md

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

Data-Structures/Array/QuickSelect.js

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+/**
+ * QuickSelect is an algorithm to find the kth smallest number
+ *
+ * Notes:
+ * -QuickSelect is related to QuickSort, thus has optimal best and average
+ *  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) {
+  return RandomizedSelect(items, 0, items.length - 1, kth)
+}
+
+function RandomizedSelect (
+  items,
+  left,
+  right,
+  i
+) {
+  if (left === right) return items[left]
+
+  const pivotIndex = RandomizedPartition(items, left, right)
+  const k = pivotIndex - left + 1
+
+  if (i === k) return items[pivotIndex]
+  if (i < k) return RandomizedSelect(items, left, pivotIndex - 1, i)
+
+  return RandomizedSelect(items, pivotIndex + 1, right, i - k)
+}
+
+function RandomizedPartition (items, left, right) {
+  const rand = getRandomInt(left, right)
+  Swap(items, rand, right)
+  return Partition(items, left, right)
+}
+
+function Partition (items, left, right) {
+  const x = items[right]
+  let pivotIndex = left - 1
+
+  for (let j = left; j < right; j++) {
+    if (items[j] <= x) {
+      pivotIndex++
+      Swap(items, pivotIndex, j)
+    }
+  }
+
+  Swap(items, pivotIndex + 1, right)
+
+  return pivotIndex + 1
+}
+
+function getRandomInt (min, max) {
+  return Math.floor(Math.random() * (max - min + 1)) + min
+}
+
+function Swap (arr, x, y) {
+  const temp = arr[x]
+  arr[x] = arr[y]
+  arr[y] = temp
+}
+
+// testing
+console.log(QuickSelect([1, 4, 2, -2, 4, 5]))