heapify.py

#!/usr/bin/python

def heapify(arr, n, i):
	largest = i # Initialize largest as root
	l = 2 * i + 1 # left = 2*i + 1
	r = 2 * i + 2 # right = 2*i + 2

# See if left child of root exists and is
# greater than root

	if l < n and arr[i] < arr[l]:
		largest = l

# See if right child of root exists and is
# greater than root

	if r < n and arr[largest] < arr[r]:
		largest = r

# Change root, if needed

	if largest != i:
		(arr[i], arr[largest]) = (arr[largest], arr[i]) # swap

# Heapify the root.

		heapify(arr, n, largest)


# The main function to sort an array of given size

def heapSort(arr):
	n = len(arr)

# Build a maxheap.
# Since last parent will be at ((n//2)-1) we can start at that location.
	for i in range(n // 2 - 1, -1, -1):
		heapify(arr, n, i)

# One by one extract elements
	for i in range(n - 1, 0, -1):
		(arr[i], arr[0]) = (arr[0], arr[i]) # swap
		heapify(arr, i, 0)


# Driver code to test above
arr = [12, 11, 13, 5, 6, 7, ]
heapSort(arr)
n = len(arr)
print('Sorted array is')
for i in range(n):
	print(arr[i])