Quicksort is a divide-and-conquer sorting algorithm in which division is dynamically carried out (as opposed to static division in Mergesort).

The three steps of Quicksort are as follows:

Divide: Rearrange the elements and split the array into two subarrays and an element in between such that so that each element in the left subarray is less than or equal the middle element and each element in the right subarray is greater than the middle element.

Conquer: Recursively sort the two subarrays.

Combine: None.

Algorithm

# choose pivot

swap a[1,rand(1,n)]

# 2-way partition

k = 1
for i = 2:n, if a[i] < a[1], swap a[++k,i]
swap a[1,k]
→ invariant: a[1..k-1] < a[k] <= a[k+1..n]

# recursive sorts

sort a[1..k-1]
sort a[k+1,n]

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace QuickSort

{

class QuickSort

{

static public int Partition(int[] numbers, int left, int right)

{

int pivot = numbers[left];

while (true)

{

while (numbers[left] < pivot)

left++;

while (numbers[right] > pivot)

right--;

if (left < right)

{

int temp = numbers[right];

numbers[right] = numbers[left];

numbers[left] = temp;

}

else

{

return right;

}

}

}

static public void SortQuick(int[] arr, int left, int right)

{

// For Recusrion

if (left < right)

{

int pivot = Partition(arr, left, right);

if (pivot > 1)

SortQuick(arr, left, pivot - 1);

if (pivot + 1 < right)

SortQuick(arr, pivot + 1, right);

}

}

static void Main(string[] args)

{

Console.Write("\nProgram for sorting a numeric array using Quick Sorting");

Console.Write("\n\nEnter number of elements: ");

int max = Convert.ToInt32(Console.ReadLine());

int[] numbers = new int[max];

for (int i = 0; i < max; i++)

{

Console.Write("\nEnter [" + (i + 1).ToString() + "] element: ");

numbers[i] = Convert.ToInt32(Console.ReadLine());

}

Console.Write("Input int array : ");

Console.Write("\n");

for (int k = 0; k < max; k++)

{

Console.Write(numbers[k] + " ");

Console.Write("\n");

}

Console.WriteLine("QuickSort By Recursive Method");

SortQuick(numbers, 0, max - 1);

for (int i = 0; i < max; i++)

Console.WriteLine(numbers[i]);

Console.ReadLine();

}

}

}