From d4af68bd865cffa3a3c41305f1a165bcaf3c9b25 Mon Sep 17 00:00:00 2001
From: Lars Kotthoff <larsko@uwyo.edu>
Date: Wed, 15 Aug 2018 17:32:45 -0600
Subject: [PATCH 1/2] choose pivot from median of three elements for quicksort
---
src/algorithms/sorting/quick-sort/QuickSort.js | 18 +++++++++++++-----
.../quick-sort/__test__/QuickSort.test.js | 8 ++++----
2 files changed, 17 insertions(+), 9 deletions(-)
diff --git a/src/algorithms/sorting/quick-sort/QuickSort.js b/src/algorithms/sorting/quick-sort/QuickSort.js
index 1c63c86e8b..c1d1433247 100644
--- a/src/algorithms/sorting/quick-sort/QuickSort.js
+++ b/src/algorithms/sorting/quick-sort/QuickSort.js
@@ -14,13 +14,21 @@ export default class QuickSort extends Sort {
return array;
}
- // Init left and right arrays.
+ // Init left, center, and right arrays.
const leftArray = [];
const rightArray = [];
-
- // Take the first element of array as a pivot.
- const pivotElement = array.shift();
- const centerArray = [pivotElement];
+ const centerArray = [];
+
+ // Take the median element of first, mid, and last elements.
+ let pivotElement = array[0];
+ const mid = Math.floor(array.length / 2);
+ if ((array[mid] < array[array.length - 1] && array[mid] > array[0])
+ || (array[mid] > array[array.length - 1] && array[mid] < array[0])) {
+ pivotElement = array[mid];
+ } else if ((array[array.length - 1] < array[mid] && array[array.length - 1] > array[0])
+ || (array[array.length - 1] > array[mid] && array[array.length - 1] < array[0])) {
+ pivotElement = array[array.length - 1];
+ }
// Split all array elements between left, center and right arrays.
while (array.length) {
diff --git a/src/algorithms/sorting/quick-sort/__test__/QuickSort.test.js b/src/algorithms/sorting/quick-sort/__test__/QuickSort.test.js
index 71c1fe71e0..5215771659 100644
--- a/src/algorithms/sorting/quick-sort/__test__/QuickSort.test.js
+++ b/src/algorithms/sorting/quick-sort/__test__/QuickSort.test.js
@@ -8,10 +8,10 @@ import {
} from '../../SortTester';
// Complexity constants.
-const SORTED_ARRAY_VISITING_COUNT = 190;
-const NOT_SORTED_ARRAY_VISITING_COUNT = 62;
-const REVERSE_SORTED_ARRAY_VISITING_COUNT = 190;
-const EQUAL_ARRAY_VISITING_COUNT = 19;
+const SORTED_ARRAY_VISITING_COUNT = 66;
+const NOT_SORTED_ARRAY_VISITING_COUNT = 83;
+const REVERSE_SORTED_ARRAY_VISITING_COUNT = 66;
+const EQUAL_ARRAY_VISITING_COUNT = 20;
describe('QuickSort', () => {
it('should sort array', () => {
From 478abbadc93b7a494f7c1f039bee265d372479e0 Mon Sep 17 00:00:00 2001
From: Lars Kotthoff <larsko@uwyo.edu>
Date: Tue, 11 Dec 2018 10:49:49 -0700
Subject: [PATCH 2/2] fix pivot picking and kruskal's algorithm
---
src/algorithms/graph/kruskal/kruskal.js | 2 +-
src/algorithms/sorting/quick-sort/QuickSort.js | 12 ++++++++----
2 files changed, 9 insertions(+), 5 deletions(-)
diff --git a/src/algorithms/graph/kruskal/kruskal.js b/src/algorithms/graph/kruskal/kruskal.js
index dd9968a001..3c44a2ad36 100644
--- a/src/algorithms/graph/kruskal/kruskal.js
+++ b/src/algorithms/graph/kruskal/kruskal.js
@@ -24,7 +24,7 @@ export default function kruskal(graph) {
*/
compareCallback: (graphEdgeA, graphEdgeB) => {
if (graphEdgeA.weight === graphEdgeB.weight) {
- return 1;
+ return 0;
}
return graphEdgeA.weight <= graphEdgeB.weight ? -1 : 1;
diff --git a/src/algorithms/sorting/quick-sort/QuickSort.js b/src/algorithms/sorting/quick-sort/QuickSort.js
index c1d1433247..c3c94361c8 100644
--- a/src/algorithms/sorting/quick-sort/QuickSort.js
+++ b/src/algorithms/sorting/quick-sort/QuickSort.js
@@ -22,11 +22,15 @@ export default class QuickSort extends Sort {
// Take the median element of first, mid, and last elements.
let pivotElement = array[0];
const mid = Math.floor(array.length / 2);
- if ((array[mid] < array[array.length - 1] && array[mid] > array[0])
- || (array[mid] > array[array.length - 1] && array[mid] < array[0])) {
+ if ((this.comparator.lessThan(array[mid], array[array.length - 1])
+ && this.comparator.greaterThan(array[mid], array[0]))
+ || (this.comparator.greaterThan(array[mid], array[array.length - 1])
+ && this.comparator.lessThan(array[mid], array[0]))) {
pivotElement = array[mid];
- } else if ((array[array.length - 1] < array[mid] && array[array.length - 1] > array[0])
- || (array[array.length - 1] > array[mid] && array[array.length - 1] < array[0])) {
+ } else if ((this.comparator.lessThan(array[array.length - 1], array[mid])
+ && this.comparator.greaterThan(array[array.length - 1], array[0]))
+ || (this.comparator.greaterThan(array[array.length - 1], array[mid])
+ && this.comparator.lessThan(array[array.length - 1], array[0]))) {
pivotElement = array[array.length - 1];
}