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# Quicksort

Posted on 1998-03-25
Medium Priority
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How would yo modify a quicksort algorithm so that it would run in O(N * log2N) time if the initial array was sorted or random?    Be sure to explain why your modification makes a difference.   Note:  O(N * log2N) <- 2 is a subscript
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Question by:gokou
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jos010697 earned 400 total points
ID: 1257808
Average big-oh (read: for random input) of the quicksort algorithm is n*log2(n)
already, prepending a single sweep (big-oh == n) checking whether or not
the input is sorted already doesn't change the overall big-oh value ...

kind regards,

Jos aka jos@and.nl
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Author Comment

ID: 1257809
Jos, I'm not sure if your answer is correct.  I also looked up the word prepending in Webster's Dictionary and I couldn't find the definition for this word.  Can you explain what you mean by prepending?

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Expert Comment

ID: 1257810
I apologize if I didn't make myself clear enough; I meant: before starting the
original quicksort algorithm, a simple check could be performed figuring
out whether or not the input is sorted already (ascending or descending),

if so, the quicksort algorithm does not have do be started, otherwise,
a simple quicksort is started; the big-oh of this check+quicksort is

O(n)+O(n*2log(n)) == O(n*2log(n))

kind regards,

Jos aka jos@and.nl

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LVL 84

Expert Comment

ID: 1257811
Nor does the pre-check change the n² worst case behavior for pathologically almost sorted input.
There are ways to select a near median pivot element in O(n) time which guarantee O(n*log(n) worst case behavior,
but these can affect the constant factor that makes quicksort so useful in the average case.
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