choose pivot from median of three elements for quicksort

[larskotthoff]

The method for choosing the pivot as implemented at the moment results in degenerate behaviour when the array is already sorted or reverse sorted. A better way is to choose the median of three elements (first, mid, last) as the pivot. See Robert Sedgewick. Implementing Quicksort Programs. Commun. ACM, 1(10):847–857, October 1978.

[larskotthoff]

Test failures seem to be unrelated? Not sure what's going on there.

[orist22]

I think it is critical to choose appropriate pivot. If you fix the conflict, it would be really helpful!

[orist22]

The code bot says that you exceeded the maximum size of array. I hope it would be helpful!

[codecov]

Codecov Report

Merging #171 into master will not change coverage.
The diff coverage is 100%.

@@ Coverage Diff @@## master #171 +/- ## ===================================== Coverage 100% 100% ===================================== Files 147 147 Lines 2590 2595 +5 Branches 432 434 +2 ===================================== + Hits 2590 2595 +5

Impacted Files	Coverage Δ
src/algorithms/sorting/quick-sort/QuickSort.js	100% <100%> (ø)	⬆️
src/algorithms/graph/kruskal/kruskal.js	100% <100%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #171 into master will decrease coverage by 0.41%.
The diff coverage is 100%.

@@ Coverage Diff @@## master #171 +/- ## ========================================== - Coverage 100% 99.58% -0.42%  ========================================== Files 147 128 -19 Lines 2590 2429 -161 Branches 432 413 -19 ========================================== - Hits 2590 2419 -171 - Misses 0 9 +9 - Partials 0 1 +1

Impacted Files	Coverage Δ
src/algorithms/sorting/quick-sort/QuickSort.js	100% <100%> (ø)	⬆️
src/algorithms/graph/kruskal/kruskal.js	44.44% <0%> (-55.56%)	⬇️
...algorithms/sorting/selection-sort/SelectionSort.js	100% <0%> (ø)	⬆️
...hms/math/euclidean-algorithm/euclideanAlgorithm.js	100% <0%> (ø)	⬆️
src/data-structures/stack/Stack.js	100% <0%> (ø)	⬆️
src/utils/comparator/Comparator.js	100% <0%> (ø)	⬆️
src/algorithms/sorting/bubble-sort/BubbleSort.js	100% <0%> (ø)	⬆️
...rc/algorithms/search/binary-search/binarySearch.js	100% <0%> (ø)	⬆️
src/data-structures/trie/Trie.js	100% <0%> (ø)	⬆️
src/data-structures/heap/MinHeap.js	100% <0%> (ø)	⬆️
... and 32 more

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[larskotthoff]

Should be fixed -- I wasn't choosing pivots the right way, and it turned out Kruskal's algorithm was broken...

[larskotthoff]

@trekhleb Could you have a look please? This makes quicksort better and fixes a bug in Kruskal's algorithm that breaks it in some cases.

[RayJune]

The method for choosing the pivot as implemented at the moment results in degenerate behaviour when the array is already sorted or reverse sorted. A better way is to choose the median of three elements (first, mid, last) as the pivot. See Robert Sedgewick. Implementing Quicksort Programs. Commun. ACM, 1(10):847–857, October 1978.

What a nice improvement. @larskotthoff Thanks a lot.

The fact that this PR is not merged in is a loss for the repository.

[lazarljubenovic]

The repo is mostly dead.