Python Heap Sort Program

Heap Sort

In this tutorial, we will learn how to write a program for Heap Sort algorithm.

Program

In the following program, we write a function heap_sort() that takes a list of numbers, and sorts them in ascending order.

We also show how to use list.reverse() method, to reverse the order of a default sorted list, to get descending order.

Python Program

``````def heap_sort(alist):
build_max_heap(alist)
for i in range(len(alist) - 1, 0, -1):
alist[0], alist[i] = alist[i], alist[0]
max_heapify(alist, index=0, size=i)

def parent(i):
return (i - 1)//2

def left(i):
return 2*i + 1

def right(i):
return 2*i + 2

def build_max_heap(alist):
length = len(alist)
start = parent(length - 1)
while start >= 0:
max_heapify(alist, index=start, size=length)
start = start - 1

def max_heapify(alist, index, size):
l = left(index)
r = right(index)
if (l < size and alist[l] > alist[index]):
largest = l
else:
largest = index
if (r < size and alist[r] > alist[largest]):
largest = r
if (largest != index):
alist[largest], alist[index] = alist[index], alist[largest]
max_heapify(alist, largest, size)

nlist = input('Enter the list of numbers separated by space\n').split()
nlist = [int(x) for x in nlist]
heap_sort(nlist)
print('Sorted List after Heap Sort, in Ascending Order\n', nlist)
nlist.reverse()
print('Sorted List after Heap Sort, in Descending Order\n', nlist)``````
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Output

``````Enter the list of numbers separated by space
1 74 96 5 42 63
Sorted List after Heap Sort, in Ascending Order
[1, 5, 42, 63, 74, 96]
Sorted List after Heap Sort, in Descending Order
[96, 74, 63, 42, 5, 1]``````
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