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-   -   QuickSort in C# (http://www.go4expert.com/articles/quicksort-c-sharp-t24409/)

Smoke6969 6Jan2011 17:01

QuickSort in C#
 
A few days ago I had an interview on a c# programmer vacancy (already received a refuse letter :happy: ) On the interview I was asked to implement a QuickSort algorithm and I couldn't do that, so now I want to help you not to repeat my mistake and get the understanding of this algorithm before you'll need it.

Simple Algorithm

  1. Pick an element from the list (usually the most left element is used). This element will serve as a pivot point
  2. Reorder the list so that all elements with values less than the pivot come before the pivot, while all elements with values greater than the pivot come after it (equal values can go either way). This is called the partition operation.
  3. Recursively repeat the algorithm for both halves of the original array.
That's it. Pretty clear as I think.

Pseudo Code



Here is the Pseudo code for better understanding:

Code:

function quicksort(array)
    var list less, greater
    if length(array) ≤ 1
        return array
    select and remove a pivot value pivot from array
    for each x in array
        if x ≤ pivot then append x to less
        else append x to greater
    return concatenate(quicksort(less), pivot, quicksort(greater))

C# Implementation



That's enough for theory, here is my implementation of this in C#:

Code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace sortQuick
{
    class quickSort
    {
        // this is our array to sort
        private int[] arr = new int[20];

        // this holds a number of elements in array
        private int len;

        // Quick Sort Algorithm
        public void QuickSort()
        {
            sort(0, len - 1);
        }

        public void sort(int left, int right)
        {
            int pivot, l_holder, r_holder;

            l_holder = left;
            r_holder = right;
            pivot = arr[left];

            while (left < right)
            {
                while ((arr[right] >= pivot) && (left < right))
                {
                    right--;
                }

                if (left != right)
                {
                    arr[left] = arr[right];
                    left++;
                }

                while ((arr[left] <= pivot) && (left < right))
                {
                    left++;
                }

                if (left != right)
                {
                    arr[right] = arr[left];
                    right--;
                }
            }

            arr[left] = pivot;
            pivot = left;
            left = l_holder;
            right = r_holder;

            if (left < pivot)
            {
                sort(left, pivot - 1);
            }

            if (right > pivot)
            {
                sort(pivot + 1, right);
            }
        }

        public static void Main()
        {
            quickSort q_Sort = new quickSort();

            int[] arr = {4,3,1,4,6,7,5,4,3,5,6,87,8};
            q_Sort.arr = arr;
            q_Sort.len = q_Sort.arr.Length;

            // Sort the array
            q_Sort.QuickSort();

            for (int j = 0; j < q_Sort.len; j++)
            {
                Console.WriteLine(q_Sort.arr[j]);
            }
            Console.ReadKey();
        }
    }
}



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