Name:
Lesson: Quick Sort
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Lesson: Quick Sort – A
Usually
Fast
Sorting Algorithm
In this lesson we introduce quick sort. Quick sort, as the name suggests, is
another fast sorting algorithm. Quick sort is also recursive, which will give us the
opportunity to once again use the recursive analysis techniques introduced in the
previous lesson. Quick sort has differing best and worst case execution times,
similar in this regard to insertion sort. Finally, quick sort presents an unusual
contrast to the merge sort algorithm.
The quick sort algorithm is in one sense similar to, and in another sense very
different from the merge sort algorithm. Both work by breaking an array into two
parts, recursively sorting the two pieces, and putting them back together to form
the final result.
Where they differ is in the
way that the array is
broken into pieces, and
hence the way that they
are put back together.
Merge sort devotes very
little effort to the breaking
apart phase, simply
selecting the first half of
the array for the first list,
and the second half for the
second. This means that relatively little can be said about the two sub-arrays,
and significant work must be performed to merge them back together.
Quick sort, on the other
hand, spends more time on
the task of breaking apart.
Quick sort selects one
element, which is called the
pivot
. It then divides the
array into two sections with
the following property:
every element in the first
half is smaller than or equal
to the pivot value, while
every element in the
second half is larger than or equal to the pivot. Notice that this property does not
guarantee sorting. Each of the smaller arrays must then be sorted (by recursively
calling quick sort). But once sorted, this property makes putting the two sections
