quicksort : Java Glossary

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©1996-2017 Roedy Green of Canadian Mind Products

quicksort
C.A.R. Hoare’s recursive sorting technique. It works with a pivot element, moving all keys smaller than the pivot to one side and all the keys bigger to the other. Then it recursively sorts each half. QuickSort can be pathologically slow if the data are already ordered. In Java, QuickSort is slower than either HeapSort or RadixSort. Typical QuickSort implementations are unstable since they scramble keys to avoid pathological pre-orderings. Free Java source code is available from Roedy Green at Canadian Mind Products.

Oddly, the Haskell version of Quicksort is probably the easiest to understand:

qsort []     = []
qsort (x:xs) = qsort (filter (< x) xs) ++ [x] ++ qsort (filter (>= x) xs)
The first line reads: the result of sorting an empty list ([]) is an empty list.

The second line reads: to sort a list whose first element is x and the rest of which is called xs, sort the elements of xs that are less than x, sort the elements of xs that are greater than or equal to x and concatenate (++) the results with x sandwiched in the middle.

To learn more about quicksort’s behaviour see Eppstein’s paper. QuickSort source code download..

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