December 28, 2017

The Quick Sort Algorithm

Quick Sort is one of the most popular sorting algorithms based on divide et impera strategy, resulting in an O(n log n) complexity. The algorithm starts by picking an item, called pivot, and moving all smaller items before it, while all greater elements after it. This is the main quick sort operation, called partition, recursively repeated on lesser and greater sub lists until their size is one or zero-in which case the list is implicitly sorted.

Choosing an appropriate pivot, as for example the median element is funda- mental for avoiding the drastically reduced performance of O(n2).

Quick Sort, as the name suggests, sorts any list very quickly. It is not a stable search, but it is very fast and requires very less additional space. As stated above, It is based on the rule of Divide and Conquer(also called partition-exchange sort). This algorithm divides the list into following three main parts :

  1. Elements less than the Pivot element
  2. Pivot element(Central element)
  3. Elements greater than the pivot element

In the list of elements, mentioned in below example, I have taken 25 as pivot.



algorithm QuickSort(list)
  Pre: list 6 = 
  Post: list has been sorted into values of ascending order
  if list.Count = 1 // already sorted
    return list
  end if
  pivot MedianValue(list)
  for i  0 to list.Count1
    if list[i] = pivot
    end if
    if list[i] < pivot
    end if
    if list[i] > pivot
    end if
  end for
  return Concatenate(QuickSort(less), equal, QuickSort(greater))
end Quicksort

That’s it for this one !!!