Java 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 first find the maximum element and place it at the end. We repeat the same process for the remaining element. 

Java

// Java program for implementation of Heap Sort
public class HeapSort {
    public void sort(int arr[])
    {
        int n = arr.length;

        // 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
            int temp = arr[0];
            arr[0] = arr[i];
            arr[i] = temp;

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

    // 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
        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) {
            int swap = arr[i];
            arr[i] = arr[largest];
            arr[largest] = swap;

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

    /* A utility function to print array of size n */
    static void printArray(int arr[])
    {
        int n = arr.length;
        for (int i = 0; i < n; ++i)
            System.out.print(arr[i] + " ");
        System.out.println();
    }

    // Driver program
    public static void main(String args[])
    {
        int arr[] = { 12, 11, 13, 5, 6, 7 };
        int n = arr.length;

        HeapSort ob = new HeapSort();
        ob.sort(arr);

        System.out.println("Sorted array is");
        printArray(arr);
    }
}

Time Complexity : O(N log N), here N is number of elements in array.

Auxiliary Space : O(1), since no extra space used.

Approach Name: Heap Sort using STL in Java

Steps:

1. Convert the input array into a max heap using the STL priority queue.
2. Remove the top element of the max heap and place it at the end of the array.
3. Repeat step 2 for all the remaining elements in a heap.

Java

import java.util.*;

public class HeapSortUsingSTL {

    // Function to perform the heap sort
    public static void heapSort(int[] arr)
    {
        PriorityQueue<Integer> maxHeap
            = new PriorityQueue<>(
                Collections.reverseOrder());
        for (int i = 0; i < arr.length; i++) {
            maxHeap.offer(arr[i]);
        }
        for (int i = arr.length - 1; i >= 0; i--) {
            arr[i] = maxHeap.poll();
        }
    }

    // Driver Code
    public static void main(String[] args)
    {
        int[] arr = { 60, 20, 40, 70, 30, 10 };
        System.out.println("Before Sorting: "
                           + Arrays.toString(arr));
        heapSort(arr);
        System.out.println("After Sorting: "
                           + Arrays.toString(arr));
    }
}

Output

Before Sorting: [60, 20, 40, 70, 30, 10]
After Sorting: [10, 20, 30, 40, 60, 70]

Time Complexity: O(n log n)
Auxiliary Space: O(n)

Please refer complete article on Heap Sort for more details!