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Analyze the best average and worst-case complexity of quicksort

#1
01-07-2024, 11:01 AM
You see quicksort grabs a pivot and splits things quick. I always notice how the best case hits when every split stays even. You watch the array break into equal halves each pass. And recursion then runs in balanced layers that stack up smooth. The time drops to n log n because partitions avoid any waste. But you know the math hides behind those even cuts. Perhaps random choices push it there often enough. I tried it on sample data and it flew by fast.

Or think about how average case mirrors that best spot most times. You pick pivots without bias and the splits hover around half. I figure the expected depth stays logarithmic overall. And the total work sums to n log n again with high probability. But uneven bits pop up here and there without killing speed. You end up with the same bound because bad splits cancel out across calls. Maybe probability keeps things centered on that log factor. I ran tests myself and averages stayed tight. The process avoids deep stacks unless luck turns sour.

Worst case drags when the pivot always lands at an end. You see one side empties while the other swallows everything. I hate how recursion then peels off one element at a time. And the calls pile up to n levels deep. That pushes work into quadratic territory with n squared over two steps. But you avoid it by swapping to median picks or random ones. Perhaps bad data like sorted lists trigger it every run. I saw it crawl on reverse order inputs once. The comparisons explode without balance to help.

Now the partition step itself costs linear time per level. You count swaps and checks that hit every element. I break it down as the real bottleneck in bad scenarios. And best or average keeps those levels few. But worst stretches them out fully. You mix in three way splits sometimes to dodge repeats. Perhaps median of three tweaks the odds your way. I prefer it for real code because it cuts extremes. The whole thing stays simple yet powerful when tuned right.

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ProfRon
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Analyze the best average and worst-case complexity of quicksort

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