#include<stdio.h>
/* Logic: This is divide and conquer algorithm like Merge Sort. Unlike Merge Sort this does not require
          extra space. So it sorts in place. Here dividing step is chose a pivot and parition the array
          such that all elements less than or equal to pivot are to the left of it andd all the elements  
          which  are greater than or equal to the pivot are to the right of it. Recursivley sort the left 
          and right parts.
*/

void QuickSort(int *array, int from, int to)
{
        if(from>=to)return;
        int pivot = array[from]; /*Pivot I am chosing is the starting element */
        /*Here i and j are such that in the array all elemnts a[from+1...i] are less than pivot,
          all elements a[i+1...j] are greater than pivot and the elements a[j+1...to] are which 
          we have not seen which is like shown below.
          ________________________i_________________j___________
          |pivot|....<=pivot......|....>=pivot......|.........|
          If the new element we encounter than >=pivot the above variant is still satisfied.
          If it is less than pivot we swap a[j] with a[i+1].
         */
        int i = from, j, temp;
        for(j = from + 1;j <= to;j++)
        {
                if(array[j] < pivot) 
                {
                        i = i + 1;
                        temp = array[i];
                        array[i] = array[j];
                        array[j] = temp;
                }
        }
        /* Finally The picture is like shown below
          _______________________i____________________
          |pivot|....<=pivot......|....>=pivot......|
        */
        temp = array[i];
        array[i] = array[from];
        array[from] = temp;
        /* So we the array is now 
          ____________________i______________________
          |...<=pivot......|pivot|....>=pivot......|
        */
        /*Recursively sort the two sub arrays */
        QuickSort(array,from,i-1);
        QuickSort(array,i+1,to);
}
int main()
{
        int number_of_elements;
        scanf("%d",&number_of_elements);
        int array[number_of_elements]; 
        int iter;
        for(iter = 0;iter < number_of_elements;iter++)
        {
                scanf("%d",&array[iter]);
        }
        /* Calling this functions sorts the array */
        QuickSort(array,0,number_of_elements-1); 
        for(iter = 0;iter < number_of_elements;iter++)
        {
                printf("%d ",array[iter]);
        }
        printf("\n");
        return 0;
}

Quick Sort - Java Program Source Code

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/* Logic: This is divide and conquer algorithm like Merge Sort. Unlike Merge Sort this does not require
          extra space. So it sorts in place. Here dividing step is chose a pivot and parition the array
          such that all elements less than or equal to pivot are to the left of it andd all the elements  
          which  are greater than or equal to the pivot are to the right of it. Recursivley sort the left 
          and right parts.
*/
import java.io.*;
class QuickSort
{
void QuickSort(int array[], int from, int to)
{
        if(from>=to)return;
        int pivot = array[from]; /*Pivot I am chosing is the starting element */
        /*Here i and j are such that in the array all elemnts a[from+1...i] are less than pivot,
          all elements a[i+1...j] are greater than pivot and the elements a[j+1...to] are which 
          we have not seen which is like shown below.
          ________________________i_________________j___________
          |pivot|....<=pivot......|....>=pivot......|.........|
          If the new element we encounter than >=pivot the above variant is still satisfied.
          If it is less than pivot we swap a[j] with a[i+1].
         */
        int i = from, j, temp;
        for(j = from + 1;j <= to;j++)
        {
                if(array[j] < pivot) 
                {
                        i = i + 1;
                        temp = array[i];
                        array[i] = array[j];
                        array[j] = temp;
                }
        }
        /* Finally The picture is like shown below
          _______________________i____________________
          |pivot|....<=pivot......|....>=pivot......|
        */
        temp = array[i];
        array[i] = array[from];
        array[from] = temp;
        /* So we the array is now 
          ____________________i______________________
          |...<=pivot......|pivot|....>=pivot......|
        */
        /*Recursively sort the two sub arrays */
        QuickSort(array,from,i-1);
        QuickSort(array,i+1,to);
}
int main()throws IOException
{
    BufferedReader in=new BufferedReader(new InputStreamReader(System.in));
        int number_of_elements;
        System.out.println("Enter the number of elements");
        number_of_elements=Integer.parseInt(in.readLine());
        int array[]=new int[number_of_elements]; 
        int iter;
        System.out.println("Enter the elements one by one");
        for(iter = 0;iter < number_of_elements;iter++)
        {
                array[iter]=Integer.parseInt(in.readLine());        }
        /* Calling this functions sorts the array */
        QuickSort(array,0,number_of_elements-1); 
        for(iter = 0;iter < number_of_elements;iter++)
        {
               System.out.print(array[iter]+"\t");
        }
        System.out.print("\n");
        return 0;
}
}