Merge pull request #542 from achudinovskyh/Add-Quickselect

Added QuickSelect  Algorithm

DIRECTORY.md

@@ -185,6 +185,7 @@
   * [InterpolationSearch](https://github.com/TheAlgorithms/Javascript/blob/master/Search/InterpolationSearch.js)
   * [JumpSearch](https://github.com/TheAlgorithms/Javascript/blob/master/Search/JumpSearch.js)
   * [LinearSearch](https://github.com/TheAlgorithms/Javascript/blob/master/Search/LinearSearch.js)
+  * [QuickSelect](https://github.com/TheAlgorithms/Javascript/blob/master/Search/QuickSelectSearch.js)
   * [StringSearch](https://github.com/TheAlgorithms/Javascript/blob/master/Search/StringSearch.js)
 
 ## Sorts

Search/QuickSelectSearch.js

@@ -0,0 +1,55 @@
+/*
+ * Places the `k` smallest elements in `array` in the first `k` indices: `[0..k-1]`
+ * Modifies the passed in array *in place*
+ * Returns a slice of the wanted elements for convenience
+ * Efficient mainly because it never performs a full sort.
+ *
+ * The only guarantees are that:
+ *
+ * - The `k`th element is in its final sort index (if the array were to be sorted)
+ * - All elements before index `k` are smaller than the `k`th element
+ *
+ * [Reference](http://en.wikipedia.org/wiki/Quickselect)
+ */
+function quickSelectSearch (array, k) {
+  if (!array || array.length <= k) {
+    throw new Error('Invalid arguments')
+  }
+
+  let from = 0
+  let to = array.length - 1
+  while (from < to) {
+    let left = from
+    let right = to
+    const pivot = array[Math.ceil((left + right) * 0.5)]
+
+    while (left < right) {
+      if (array[left] >= pivot) {
+        const tmp = array[left]
+        array[left] = array[right]
+        array[right] = tmp
+        --right
+      } else {
+        ++left
+      }
+    }
+
+    if (array[left] > pivot) {
+      --left
+    }
+
+    if (k <= left) {
+      to = left
+    } else {
+      from = left + 1
+    }
+  }
+  return array
+}
+
+/* ---------------------------------- Test ---------------------------------- */
+
+const arr = [1121111, 21, 333, 41, 5, 66, 7777, 28, 19, 11110]
+console.log(quickSelectSearch(arr, 5)) // [ 19, 21, 28, 41, 5, 66, 333, 11110, 1121111, 7777 ]
+console.log(quickSelectSearch(arr, 2)) // [ 19, 5, 21, 41, 28, 333, 11110, 1121111, 7777, 66 ]
+console.log(quickSelectSearch(arr, 7)) // [ 19, 5, 21, 41, 28, 66, 333, 7777, 11110, 1121111 ]