heap sort and reverse heap sort with max/min heap

Question

I just want to check if I understood what the professor and online ressources said.

For a heapSort algorithm, the index of the first element starts at 0.

For a max heap, percolate down should swap the maximum child with its parent if the child is bigger than the parent, so something like (this is for an assignement so I tried to post as little code as possible):

def percolateDownMaxHeap( array, hole, length):
    while hole * 2 < size - 1:
        left = 2*hole + 1
        right = left + 1

        max = left

        if (left < size - 1) and (array[right] > array[left]):
            max = right

        if (array[max] > array[hole]):
            swap(array, hole, max)

        else:
            break

        hole = max

So in the end, the maximum element should be at index 0.

If this is correct, what I don't understand is the heapSort implementation:

def heapSort(array):
    i = len(array) / 2
    while i >= 0:
        percolateDownMaxHeap( array, i, len(array))
        i -= 1

    i = len(array) - 1

    while i > 0:
        swap( array, 0, i );
        percolateDownMaxHeap( array, i, len(array))
        i -= 1

Isn't percolate down in a max heap supposed to put the largest element at index 0? In this case why not use percolate down on a min heap? The code works but I'm not sure if I got it right or if I implemented a max heap instead of a min heap

Thanks

David Eisenstat · Accepted Answer · 2020-03-16 19:59:48Z

2

Isn't percolate down in a max heap supposed to put the largest element at index 0?

It does. Then your code swaps it past the heap, so each successive swap places the next largest element where it belongs.

In this case why not use percolate down on a min heap?

That would get the min element in the right position, but the heap still needs that position in the array, so it's not clear what to do for the next element.

answered Mar 16, 2020 at 19:59

David Eisenstat

65.7k7 gold badges66 silver badges128 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

heap sort and reverse heap sort with max/min heap

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related