heap_sort.py

# This program is a comparison based sorting technique.
# It is similar to selection sort in the sense that it first identifies the maximum element,
# and places it at the end. We repeat the process until the list is sorted.
# The sort algorithm has a time complexity of O(nlogn)


def refineHeap(arr, n, i):
    # Initialize the largest entry as the root of the heap
    largest = i
    left = 2 * i + 1
    right = 2 * i + 2

    # If the left child exists and it is larger than largest, replace it
    if left < n and arr[largest] < arr[left]:
        largest = left

    # Perform the same operation for the right hand side of the heap
    if right < n and arr[largest] < arr[right]:
        largest = right

    # Change root if the largest value changed
    if largest != i:
        arr[i], arr[largest] = arr[largest], arr[i]

        # Repeat the process until the heap is fully defined
        refineHeap(arr, n, largest)


# Main function
def heapSort(arr):
    n = len(arr)

    # Make a heap
    for i in range(n // 2 - 1, -1, -1):
        refineHeap(arr, n, i)

    # Extract elements individually
    for i in range(n - 1, 0, -1):
        # Fancy notation for swapping two values in an array
        arr[i], arr[0] = arr[0], arr[i]
        refineHeap(arr, i, 0)


# Code that will run on start
arr = [15, 29, 9, 3, 16, 7, 66, 4]
print("Unsorted Array: ", arr)
heapSort(arr)
n = len(arr)
print("Sorted array: ", arr)