This worst-case performance can happen if the list is already in is in sorted order and the pivot is picked by choosing the first element of the list as the pivot.

In my data structures and algorithm analysis class, they told us to select one of the elements at random to use as the pivot. This adds a small amount of overhead (the amount of time needed to pick a random number each iteration) to the average-case scenerio, but it basically eliminates the worst-case scenerio, effectively transforming it into an average-case scenerio. "random number" here can be anything that can pass as random. If your system clock has good enough precision, you can just grab that. The key thing is that you won't be picking the same element every iteration -- sometimes an early element, sometimes a late one, sometimes a middle one. So it makes no real difference how the list is sorted initially.

This is of course all moot now; these days we just use Perl's built-in sort.


$;=sub{$/};@;=map{my($a,$b)=($_,$;);$;=sub{$a.$b->()}} split//,".rekcah lreP rehtona tsuJ";$\=$ ;->();print$/

In reply to Re: Shift, Pop, Unshift and Push with Impunity! by jonadab
in thread Shift, Pop, Unshift and Push with Impunity! by lhoward

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