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Heapsort is a comparison-based sorting algorithm, and is part of the selection sort family. Although somewhat slower in practice on most machines than a well implemented quicksort, it has the advantage of a more favorable worst-case O(n log n) runtime. Heapsort is an in-place algorithm, but is not a stable sort.

Overview
Heapsort begins by building a heap out of the data set, and then removing the largest item and placing it at the end of the partially sorted array. After removing the largest item, it reconstructs the heap, removes the largest remaining item, and places it in the next open position from the end of the partially sorted array. This is repeated until there are no items left in the heap and the sorted array is full. Elementary implementations require two arrays - one to hold the heap and the other to hold the sorted elements.

Heapsort inserts the input list elements into a binary heap data structure. The largest value (in a max-heap) or the smallest value (in a min-heap) are extracted until none remain, the values having been extracted in sorted order. The heap's invariant is preserved after each extraction, so the only cost is that of extraction.

During extraction, the only space required is that needed to store the heap. To achieve constant space overhead, the heap is stored in the part of the input array not yet sorted. (The storage of heaps as arrays is diagrammed at Binary heap.)

Heapsort uses two heap operations: insertion and root deletion. Each extraction places an element in the last empty location of the array. The remaining prefix of the array stores the unsorted elements.

Example
Let { 6, 5, 3, 1, 8, 7, 2, 4 } be the list that we want to sort from the smallest to the largest. 1. Build the heap

2. Sorting.

Variations

 * The most important variation to the simple variant is an improvement by R. W. Floyd that, in practice, gives about a 25% speed improvement by using only one comparison in each siftup run, which must be followed by a siftdown for the original child. Moreover, it is more elegant to formulate. Heapsort's natural way of indexing works on indices from 1 up to the number of items. Therefore the start address of the data should be shifted such that this logic can be implemented avoiding unnecessary +/- 1 offsets in the coded algorithm.
 * Ternary heapsort uses a ternary heap instead of a binary heap; that is, each element in the heap has three children. It is more complicated to program, but does a constant number of times fewer swap and comparison operations. This is because each step in the shift operation of a ternary heap requires three comparisons and one swap, whereas in a binary heap two comparisons and one swap are required. The ternary heap does two steps in less time than the binary heap requires for three steps, which multiplies the index by a factor of 9 instead of the factor 8 of three binary steps. Ternary heapsort is about 12% faster than the simple variant of binary heapsort.
 * The smoothsort algorithm is a variation of heapsort developed by Edsger Dijkstra in 1981. Like heapsort, smoothsort's upper bound is O(n log n). The advantage of smoothsort is that it comes closer to O(n) time if the  input is already sorted to some degree, whereas heapsort averages O(n log n) regardless of the initial sorted state. Due to its complexity, smoothsort is rarely used.
 * Levcopoulos and Petersson describe a variation of heapsort based on a Cartesian tree that does not add an element to the heap until smaller values on both sides of it have already been included in the sorted output. As they show, this modification can allow the algorithm to sort more quickly than O(n log n) for inputs that are already nearly sorted.
 * Ingo Wegener describes a bottom up version of heapsort that replaces siftdown with an alternative that reduces the worst case from 2n log(n) to 1.5n log(n) and is claimed to perform better than some versions of quicksort.

Comparison with other sorts
Heapsort primarily competes with quicksort, another very efficient general purpose nearly-in-place comparison-based sort algorithm.

Quicksort is typically somewhat faster, due to better cache behavior and other factors, but the worst-case running time for quicksort is O(n2), which is unacceptable for large data sets and can be deliberately triggered given enough knowledge of the implementation, creating a security risk. See quicksort for a detailed discussion of this problem, and possible solutions.

Thus, because of the O(n log n) upper bound on heapsort's running time and constant upper bound on its auxiliary storage, embedded systems with real-time constraints or systems concerned with security often use heapsort.

Heapsort also competes with merge sort, which has the same time bounds, but requires Ω(n) auxiliary space, whereas heapsort requires only a constant amount. Heapsort also typically runs more quickly in practice on machines with small or slow data caches. On the other hand, merge sort has several advantages over heapsort:
 * Like quicksort, merge sort on arrays has considerably better data cache performance, often outperforming heapsort on a modern desktop PC, because it accesses the elements in order.
 * Merge sort is a stable sort.
 * Merge sort parallelizes better; the most trivial way of parallelizing merge sort achieves close to linear speedup, while there is no obvious way to parallelize heapsort at all.
 * Merge sort can be easily adapted to operate on linked lists (with O(1) extra space ) and very large lists stored on slow-to-access media such as disk storage or network attached storage. Heapsort relies strongly on random access, and its poor locality of reference makes it very slow on media with long access times. (Note: Heapsort can also be applied to doubly linked lists with only O(1) extra space overhead)

Introsort is an interesting alternative to heapsort that combines quicksort and heapsort to retain advantages of both: worst case speed of heapsort and average speed of quicksort.

Pseudocode
The following is the "simple" way to implement the algorithm in pseudocode. Arrays are zero based and swap is used to exchange two elements of the array. Movement 'down' means from the root towards the leaves, or from lower indices to higher. Note that during the sort, the smallest element is at the root of the heap at a[0], while at the end of the sort, the largest element is in a[end]. function heapSort(a, count) is input:  an unordered array a of length count (first place a in max-heap order) heapify(a, count) end := count-1 //in languages with zero-based arrays the children are 2*i+1 and 2*i+2 while end > 0 do (swap the root(maximum value) of the heap with the last element of the heap) swap(a[end], a[0]) (put the heap back in max-heap order) siftDown(a, 0, end-1) (decrease the size of the heap by one so that the previous max value will         stay in its proper placement) end := end - 1 function heapify(a, count) is (start is assigned the index in a of the last parent node) start := count / 2 - 1 while start ≥ 0 do (sift down the node at index start to the proper place such that all nodes below          the start index are in heap order) siftDown(a, start, count-1) start := start - 1 (after sifting down the root all nodes/elements are in heap order) function siftDown(a, start, end) is input:  end represents the limit of how far down the heap to sift. root := start while root * 2 + 1 ≤ end do         (While the root has at least one child) child := root * 2 + 1       (root*2 + 1 points to the left child) swap := root       (keeps track of child to swap with) (check if root is smaller than left child) if a[swap] < a[child] swap := child (check if right child exists, and if it's bigger than what we're currently swapping with) if child+1 ≤ end and a[swap] < a[child+1] swap := child + 1 (check if we need to swap at all) if swap != root swap(a[root], a[swap]) root := swap         (repeat to continue sifting down the child now) else return

The heapify function can be thought of as building a heap from the bottom up, successively shifting downward to establish the heap property. An alternative version (shown below) that builds the heap top-down and shifts upward is conceptually simpler to grasp. This "siftUp" version can be visualized as starting with an empty heap and successively inserting elements. However, it is asymptotically slower: the "siftDown" version is O(n), and the "siftUp" version is O(n log n) in the worst case. The heapsort algorithm is O(n log n) overall using either version of heapify. function heapify(a,count) is     (end is assigned the index of the first (left) child of the root) end := 1 while end < count (sift up the node at index end to the proper place such that all nodes above          the end index are in heap order) siftUp(a, 0, end) end := end + 1 (after sifting up the last node all nodes are in heap order) function siftUp(a, start, end) is input:  start represents the limit of how far up the heap to sift. end is the node to sift up. child := end while child > start parent := floor((child - 1) ÷ 2) if a[parent] < a[child] then (out of max-heap order) swap(a[parent], a[child]) child := parent (repeat to continue sifting up the parent now) else return