C++ Program for Heap Sort

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

Implementation:

CPP

// C++ program for implementation of Heap Sort
#include <iostream>
using namespace std;

// To heapify a subtree rooted with node i which is
// an index in arr[]. n is size of heap
void heapify(int arr[], int n, int i)
{
    int largest = i; // Initialize largest as root Since we are using 0 based indexing
    int l = 2 * i + 1; // left = 2*i + 1
    int r = 2 * i + 2; // right = 2*i + 2

    // If left child is larger than root
    if (l < n && arr[l] > arr[largest])
        largest = l;

    // If right child is larger than largest so far
    if (r < n && arr[r] > arr[largest])
        largest = r;

    // If largest is not root
    if (largest != i) {
        swap(arr[i], arr[largest]);

        // Recursively heapify the affected sub-tree
        heapify(arr, n, largest);
    }
}

// main function to do heap sort
void heapSort(int arr[], int n)
{
    // Build heap (rearrange array)
    for (int i = n / 2 - 1; i >= 0; i--)
        heapify(arr, n, i);

    // One by one extract an element from heap
    for (int i = n - 1; i >= 0; i--) {
        // Move current root to end
        swap(arr[0], arr[i]);

        // call max heapify on the reduced heap
        heapify(arr, i, 0);
    }
}

/* A utility function to print array of size n */
void printArray(int arr[], int n)
{
    for (int i = 0; i < n; ++i)
        cout << arr[i] << " ";
    cout << "\n";
}

// Driver program
int main()
{
    int arr[] = { 60 ,20 ,40 ,70, 30, 10};
    int n = sizeof(arr) / sizeof(arr[0]);
  //heapify algorithm
  // the loop must go reverse you will get after analyzing manually
  // (i=n/2 -1) because other nodes/ ele's are leaf nodes
  // (i=n/2 -1) for 0 based indexing
  // (i=n/2)  for 1 based indexing
     for(int i=n/2 -1;i>=0;i--){
       heapify(arr,n,i);
   }

  cout << "After heapifying array is \n";
    printArray(arr, n);


    heapSort(arr, n);

    cout << "Sorted array is \n";
    printArray(arr, n);

  return 0;
}
//code by Prajwal Chougale

Output

After heapifying array is 
70 60 40 20 30 10 
Sorted array is 
10 20 30 40 60 70 

Time complexity : O(N*logN)
Auxiliary space: O(1)

Approach Name: Heap Sort (Using STL)

Steps:

1. Convert the input array to a vector.
2. Convert the vector into a Max Heap using the make_heap function of the STL.
3. Sort the Max Heap using the sort_heap function of the STL.

C++

#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

void heapSort(int arr[], int n)
{
    // Convert array to vector
    vector<int> v(arr, arr + n);

    // Convert vector to Max Heap
    make_heap(v.begin(), v.end());

    // Sort Max Heap
    sort_heap(v.begin(), v.end());

    // Copy sorted vector back to array
    copy(v.begin(), v.end(), arr);
}

int main()
{
    int arr[] = { 60, 20, 40, 70, 30, 10 };
    int n = sizeof(arr) / sizeof(arr[0]);

    heapSort(arr, n);

    cout << "Sorted array is \n";
    for (int i = 0; i < n; ++i)
        cout << arr[i] << " ";
    cout << endl;
}

Output

Sorted array is 
10 20 30 40 60 70 

Time Complexity: O(n*log(n)) in all cases.
Auxiliary Space: O(1) in-place sorting algorithm.

Please refer complete article on Heap Sort for more details!