how to implement quick sort algorithm in C++

Question

here is the of quick sort algorithm from the MITOcw(Introduction To Algorithms ) lecture

QUICKSORT(A,p,q)
if(p < q)
then r = PARTITION(A,p,q)
     QUICKSORT         


        

Answer 1

A much easier and clean implementation, also gives you number of minimum SWAPS in for QuickSort:

int quickSort(int[], int, int);

int partition(int[], int, int, int&);

int main()
{
    int array[] = {4, 2, 5};
    int size = sizeof(array)/sizeof(array[0]);

    /*
     first and last indices are passed
     idea is to move lower elements to the left of the list/pivot
     */

    int swaps = quickSort(array, 0, size-1);

    std::cout << "Minimum Swaps are: " << swaps << std::endl;

    for(int i = 0; i < size; i++)
    {
        std::cout << array[i] << " ";
    }
}

int quickSort(int array[], int start, int end)
{
    int swaps = 0;
    if(start < end)
    {
        int pIndex = partition(array, start, end, swaps);
        //after each call one number(the PIVOT) will be at its final position
        swaps += quickSort(array, start, pIndex-1);
        swaps += quickSort(array, pIndex+1, end);
    }
    return swaps;
}

int partition(int array[], int start, int end, int& swaps)
{
    int pivot = array[end];
    int pIndex = start;

    for(int i = start; i < end; i++)
    {
        if(array[i] <= pivot)
        {

            if(pIndex != i)
            {
                std::swap(array[i], array[pIndex]);
                swaps++;
            }
            pIndex++;
        }
    }
    if(pIndex != end)
    {
        std::swap(array[pIndex], array[end]);
        swaps++;
    }
    return pIndex;
}