From 53d51de06ef3b0ba47065febcc8d24b4e1e71158 Mon Sep 17 00:00:00 2001
From: Nathan Chin-Lue <nathanchinlue19@gmail.com>
Date: Tue, 16 Jun 2026 15:52:52 -0400
Subject: [PATCH 1/2] Use last element as QuickSort pivot

---
 src/algorithms/sorting/quick-sort/QuickSort.js    | 15 ++++++++-------
 .../sorting/quick-sort/__test__/QuickSort.test.js |  2 +-
 2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/src/algorithms/sorting/quick-sort/QuickSort.js b/src/algorithms/sorting/quick-sort/QuickSort.js
index 1c63c86e8b..026006c0f0 100644
--- a/src/algorithms/sorting/quick-sort/QuickSort.js
+++ b/src/algorithms/sorting/quick-sort/QuickSort.js
@@ -18,14 +18,12 @@ export default class QuickSort extends Sort {
     const leftArray = [];
     const rightArray = [];
 
-    // Take the first element of array as a pivot.
-    const pivotElement = array.shift();
-    const centerArray = [pivotElement];
+    // Take the last element of array as a pivot.
+    const pivotElement = array.pop();
+    const centerArray = [];
 
     // Split all array elements between left, center and right arrays.
-    while (array.length) {
-      const currentElement = array.shift();
-
+    array.forEach((currentElement) => {
       // Call visiting callback.
       this.callbacks.visitingCallback(currentElement);
 
@@ -36,7 +34,10 @@ export default class QuickSort extends Sort {
       } else {
         rightArray.push(currentElement);
       }
-    }
+    });
+
+    // Put the pivot after all equal elements that appeared before it to keep the sort stable.
+    centerArray.push(pivotElement);
 
     // Sort left and right arrays.
     const leftArraySorted = this.sort(leftArray);
diff --git a/src/algorithms/sorting/quick-sort/__test__/QuickSort.test.js b/src/algorithms/sorting/quick-sort/__test__/QuickSort.test.js
index 71c1fe71e0..819114e0a8 100644
--- a/src/algorithms/sorting/quick-sort/__test__/QuickSort.test.js
+++ b/src/algorithms/sorting/quick-sort/__test__/QuickSort.test.js
@@ -9,7 +9,7 @@ import {
 
 // Complexity constants.
 const SORTED_ARRAY_VISITING_COUNT = 190;
-const NOT_SORTED_ARRAY_VISITING_COUNT = 62;
+const NOT_SORTED_ARRAY_VISITING_COUNT = 85;
 const REVERSE_SORTED_ARRAY_VISITING_COUNT = 190;
 const EQUAL_ARRAY_VISITING_COUNT = 19;
 

From e9cad4a8f3c7021fe16e8b939607de73cb85fe5c Mon Sep 17 00:00:00 2001
From: Nathan Chin-Lue <nathanchinlue19@gmail.com>
Date: Fri, 19 Jun 2026 16:14:47 -0400
Subject: [PATCH 2/2] Fix Kruskal MST vertex order assertion

---
 src/algorithms/graph/kruskal/__test__/kruskal.test.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/algorithms/graph/kruskal/__test__/kruskal.test.js b/src/algorithms/graph/kruskal/__test__/kruskal.test.js
index bf96fa388c..6df30e8d5a 100644
--- a/src/algorithms/graph/kruskal/__test__/kruskal.test.js
+++ b/src/algorithms/graph/kruskal/__test__/kruskal.test.js
@@ -86,6 +86,6 @@ describe('kruskal', () => {
     expect(minimumSpanningTree.getWeight()).toBe(3);
     expect(minimumSpanningTree.getAllVertices().length).toBe(graph.getAllVertices().length);
     expect(minimumSpanningTree.getAllEdges().length).toBe(graph.getAllVertices().length - 1);
-    expect(minimumSpanningTree.toString()).toBe('A,B,C,D');
+    expect(minimumSpanningTree.getAllVertices().map((vertex) => vertex.getKey()).sort()).toEqual(['A', 'B', 'C', 'D']);
   });
 });