Fix Perfect pivot selecting an index outside the current partition

[B7M]

Fix Perfect pivot choosing an index outside the current partition

---------- PR DESCRIPTION BELOW THIS LINE ----------

Problem

In Perfect pivot mode, Quicksort and Quickselect used this code to find the pivot:

pivot = this.arrayData.indexOf(sorted[midIndex]);

The problem is that indexOf starts searching from the beginning of the whole array. The pivot should only be selected from the current partition, between left and right.

If the array has duplicate values, indexOf can find the same value at an earlier index outside the current partition. The algorithm then swaps the wrong element.

Because of this:

• Quicksort can finish with an unsorted array.
• Quickselect can return the wrong k-th element.

Fix

I added left as the starting position for indexOf:

-pivot = this.arrayData.indexOf(sorted[midIndex]);
+pivot = this.arrayData.indexOf(sorted[midIndex], left);

Now it only searches from the start of the current partition. The pivot value will be found there because sorted is made from the same partition.

I made this change in src/algo/Quicksort.js and src/algo/Quickselect.js. The other pivot modes were not changed.

Testing

I tested the change in the browser and checked the final arrays.

Quicksort

Input	Before	After	Expected
1,2,1,1	2 1b 1c 1a ❌	1a 1b 1c 2 ✅	1 1 1 2
3,2,2,2	3 2b 2c 2a ❌	2a 2b 2c 3 ✅	2 2 2 3
1,3,1,1	3 1b 1c 1a ❌	1a 1b 1c 3 ✅	1 1 1 3
2,1,1,1	2 1b 1c 1a ❌	1a 1b 1c 2 ✅	1 1 1 2

I also tested 5,5,5,5, 7,3,7,3,7, 9,1,9,1,9,1, 2,2,1,3,3,1, 6,6,6,2,6,9,1, 1,1,2,2,3,3, and 8,2,8,2,8,2,8. All sorted correctly after the fix. 5,3,1,4,2 (no duplicates) was correct before and after, since the bug needs duplicates to trigger. This is also why the examples in the dropdown never show the problem — they all use distinct values.

Quickselect

I tested 1,2,1,1 with k=4. The correct answer is 2.

• Before the fix, it returned 1 ❌
• After the fix, it returned 2 ✅

I also tested k=1 on 1,2,1,1 and 3,2,2,2, k=4 on 3,2,2,2, k=6 on 9,1,9,1,9,1, and k=3 on 5,3,1,4,2. All returned the correct result after the fix.

Checks

• npm run lint passed
• Prettier check passed for both changed files
• Only the Quicksort and Quickselect files were changed

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