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;

src/algorithms/sorting/quick-sort/QuickSort.js

@@ -14,13 +14,25 @@ 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 ((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 ((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];
+    }
 
     // Split all array elements between left, center and right arrays.
     while (array.length) {

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', () => {