## Heap Sort:

Heap sort is a comparison based sorting technique based on Binary Heap data structure. It is similar to selection sort where we first find the maximum element and place the maximum element at the end. We repeat the same process for remaining element.

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### Binary Heap:

Let us first define a Complete Binary Tree. A complete binary tree is a binary tree in which every level, except possibly the last, is completely filled, and all nodes are as far left as possible.

A Binary Heap is a Complete Binary Tree where items are stored in a special order such that value in a parent node is greater(or smaller) than the values in its two children nodes. The former is called as max heap and the latter is called min heap. The heap can be represented by binary tree or array.

**Heap Sort Algorithm for sorting in increasing order:**

**1.** Build a max heap from the input data.

**2.** At this point, the largest item is stored at the root of the heap. Replace it with the last item of the heap followed by reducing the size of heap by 1. Finally, heapify the root of tree.

**3.** Repeat above steps until size of heap is greater than 1.

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**How to build the heap?**

Heapify procedure can be applied to a node only if its children nodes are heapified. So the heapification must be performed in the bottom up order.

Lets understand with the help of an example:

Input data: 4, 10, 3, 5, 1 4(0) / 10(1) 3(2) / 5(3) 1(4) The numbers in bracket represent the indices in the array representation of data. Applying heapify procedure to index 1: 4(0) / 10(1) 3(2) / 5(3) 1(4) Applying heapify procedure to index 0: 10(0) / 5(1) 3(2) / 4(3) 1(4) The heapify procedure calls itself recursively to build heap in top down manner.

## Program to implement Heap Sort in C++

#include <iostream> using namespace std; void max_heapify(int *a, int i, int n) { int j, temp; temp = a[i]; j = 2*i; while (j <= n) { if (j < n && a[j+1] > a[j]) j = j+1; if (temp > a[j]) break; else if (temp <= a[j]) { a[j/2] = a[j]; j = 2*j; } } a[j/2] = temp; return; } void heapsort(int *a, int n) { int i, temp; for (i = n; i >= 2; i--) { temp = a[i]; a[i] = a[1]; a[1] = temp; max_heapify(a, 1, i - 1); } } void build_maxheap(int *a, int n) { int i; for(i = n/2; i >= 1; i--) { max_heapify(a, i, n); } } int main() { int n, i, x; cout<<"Enter no of elements of array\n"; cin>>n; int a[20]; for (i = 1; i <= n; i++) { cout<<"Enter element"<<(i)<<endl; cin>>a[i]; } build_maxheap(a,n); heapsort(a, n); cout<<"\n\nSorted Array\n"; for (i = 1; i <= n; i++) { cout<<a[i]<<endl; } return 0; }

### Sample Output:

Enter no of elements of array 5 Enter element1 3 Enter element2 8 Enter element3 9 Enter element4 3 Enter element5 2 Sorted Array 2 3 3 8 9