Problem Statement

Sort an array (or list) elements using the   quicksort   algorithm. The elements must have a   strict weak order   and the index of the array can be of any discrete type. For languages where this is not possible, sort an array of integers.

Quicksort, also known as   partition-exchange sort,   uses these steps.

The best pivot creates partitions of equal length (or lengths differing by   1). The worst pivot creates an empty partition (for example, if the pivot is the first or last element of a sorted array). The run-time of Quicksort ranges from   O(n log n)   with the best pivots, to   O(n2)   with the worst pivots, where   n   is the number of elements in the array.

This is a simple quicksort algorithm, adapted from Wikipedia. A better quicksort algorithm works in place, by swapping elements within the array, to avoid the memory allocation of more arrays. Quicksort has a reputation as the fastest sort. Optimized variants of quicksort are common features of many languages and libraries. One often contrasts quicksort with   merge sort,   because both sorts have an average time of   O(n log n). Quicksort is at one end of the spectrum of divide-and-conquer algorithms, with merge sort at the opposite end.

With quicksort, every element in the first partition is less than or equal to every element in the second partition. Therefore, the merge phase of quicksort is so trivial that it needs no mention! This task has not specified whether to allocate new arrays, or sort in place. This task also has not specified how to choose the pivot element. (Common ways to are to choose the first element, the middle element, or the median of three elements.) Thus there is a variety among the following implementations.

Let's start with the solution:

(define (quicksort l ??)
  (if (null? l)
      '()
      (append (quicksort (filter (lambda (x) (?? (car l) x)) (cdr l)) ??)
              (list (car l))
              (quicksort (filter (lambda (x) (not (?? (car l) x))) (cdr l)) ??))))
 
(print 
   (quicksort (list 1 3 5 9 8 6 4 3 2) >))
(print 
   (quicksort (iota 100) >))
(print 
   (quicksort (iota 100) <))