Choosing Different Pivot

Question

When i choose pivot the first element of list this code works but for example i want to make last element pivot and i wanna make minumum change in this code so i add swap(arr[high],arr[low]) before pivot=arr[low] i just change the value of low to high.But program stopped run.I don't know why this isn't work.

int partition(int low, int high, int arr[])
{
    int pivot = arr[low];
    int i = low;
    for (int j = low + 1; j <= high; ++j)
    {

        if (arr[j] < pivot)
        {
            ++i;
            swap(arr[j], arr[i]);
        }
    }
    swap(arr[low], arr[i]);
    return i;
}

void quickSort(int low, int high, int arr[])
{
    if (low < high)
    {
        int pivot = partition(low, high, arr);
        quickSort(low, pivot, arr);
        quickSort(pivot + 1, high, arr);
    }
}

Answer 1

i just change the value of low to high

If you mean you made this change:

    int pivot = arr[high];     // was arr[low]

then the code needs to be updated elsewhere. Typical lomuto partion scheme example:

int Partition(int arr[], int low, int high)
{
    int pivot = arr[high];
    int i = low;
    for (int j = low; j < high; ++j)
    {
        if (arr[j] < pivot)
        {
            std::swap(arr[j], arr[i]);
            ++i;
        }
    }
    std::swap(arr[i], arr[high]);
    return i;
}

void QuickSort(int arr[], int low, int high)
{
    if (low < high)
    {
        int pivot = Partition(arr, low, high);
        QuickSort(arr, low, pivot-1);
        QuickSort(arr, pivot + 1, high);
    }
}