如何使用固定枢轴将迭代快速排序更改为使用随机数据块的迭代快速排序？

Question

I found this code for quicksort with a fixed pivot. It always takes a right-hand side element of the given table as a pivot. I want it to take a random element as a pivot. I think that x is a pivot here, so I thought it was a good idea to change it to a random element from a given list, but it turns out that it is not working.

void swap ( int* a, int* b )
{
    int t = *a;
    *a = *b;
    *b = t;
}


int partition (int arr[], int l, int h)
{
    int x = arr[h];
    int i = (l - 1);
    for (int j = l; j <= h- 1; j++)
    {
        if (arr[j] <= x)
        {
            i++;
            swap (&arr[i], &arr[j]);
        }
    }
    swap (&arr[i + 1], &arr[h]);
    return (i + 1);
}

void quickSortIterative (int arr[], int l, int h)
{
    int stack[ h - l + 1 ]; 
    int top = -1;

    stack[ ++top ] = l;
    stack[ ++top ] = h;

    while ( top >= 0 )
    {
        h = stack[ top-- ];
        l = stack[ top-- ];

        int p = partition( arr, l, h );

        if ( p-1 > l )
        {
            stack[ ++top ] = l;
            stack[ ++top ] = p - 1;
        } 
        if ( p+1 < h )
        {
            stack[ ++top ] = p + 1;
            stack[ ++top ] = h;
        }
    }
}

I tried changing lines

int x = arr[h];

and

swap(&arr[i+1], &arr[j]);

to

int r = l+rand()%(h-l);
int x = arr[r];

and then

swap(&arr[i+1], &arr[r]);

but it is not sorting properly. Obviously I'm doing something wrong here. Please help.

Answer 1

Your partition function assumes the pivot is at the end of the partition. Consider moving your random pivot to the end of the partition first.

ie add

swap(&arr[r], &arr[h]);

after choosing your pivot.

Answer 2

The problem is that the 'partition' function now moves the pivot so it doesn't remain at the r index. And the function also misses the last element (at index h ). The simplest solution would be to place the pivot to the right-most position just after selecting it, and remain everything other the same: put swap(&arr[r], &arr[h]); before the first loop in partition() , and restore the last swap to swap (&arr[i + 1], &arr[h]);