heapify java code example

Example 1: max heap java

import java.util.PriorityQueue;

public class MaxHeapWithPriorityQueue {

    public static void main(String args[]) {
        // create priority queue
        PriorityQueue<Integer> prq = new PriorityQueue<>(Comparator.reverseOrder());

        // insert values in the queue
        prq.add(6);
        prq.add(9);
        prq.add(5);
        prq.add(64);
        prq.add(6);

        //print values
        while (!prq.isEmpty()) {
            System.out.print(prq.poll()+" ");
        }
    }

}

Example 2: heap in java

In Java PriorityQueue can be used as a Heap.

Min Heap
PriorityQueue<Integer> minHeap = new PriorityQueue<>();


Max Heap:
PriorityQueue<Integer> maxHeap = new PriorityQueue<>(Comparator.reverseOrder());

Example 3: heaps in java

public class BinaryHeap {
     
    private static final int d= 2;
    private int[] heap;
    private int heapSize;
     
    /**
     * This will initialize our heap with default size. 
     */
    public BinaryHeap(int capacity){
        heapSize = 0;
        heap = new int[ capacity+1];
        Arrays.fill(heap, -1);
         
    }
     
    /**
     *  This will check if the heap is empty or not
     *  Complexity: O(1)
     */
    public boolean isEmpty(){
        return heapSize==0;
    }
     
    /**
     *  This will check if the heap is full or not
     *  Complexity: O(1)
     */
    public boolean isFull(){
        return heapSize == heap.length;
    }
     
     
    private int parent(int i){
        return (i-1)/d;
    }
     
    private int kthChild(int i,int k){
        return d*i  +k;
    }
     
    /**
     *  This will insert new element in to heap
     *  Complexity: O(log N)
     *  As worst case scenario, we need to traverse till the root
     */
    public void insert(int x){
        if(isFull())
            throw new NoSuchElementException("Heap is full, No space to insert new element");
        heap[heapSize++] = x;
        heapifyUp(heapSize-1);
    }
     
    /**
     *  This will delete element at index x
     *  Complexity: O(log N)
     * 
     */
    public int delete(int x){
        if(isEmpty())
            throw new NoSuchElementException("Heap is empty, No element to delete");
        int key = heap[x];
        heap[x] = heap[heapSize -1];
        heapSize--;
        heapifyDown(x);
        return key;
    }
 
    /**
     *  This method used to maintain the heap property while inserting an element.
     *  
     */
    private void heapifyUp(int i) {
        int temp = heap[i];
        while(i>0 && temp > heap[parent(i)]){
            heap[i] = heap[parent(i)];
            i = parent(i);
        }
        heap[i] = temp;
    }
     
    /**
     *  This method used to maintain the heap property while deleting an element.
     *  
     */
    private void heapifyDown(int i){
        int child;
        int temp = heap[i];
        while(kthChild(i, 1) < heapSize){
            child = maxChild(i);
            if(temp < heap[child]){ heap[i] = heap[child]; }else break; i = child; } heap[i] = temp; } private int maxChild(int i) { int leftChild = kthChild(i, 1); int rightChild = kthChild(i, 2); return heap[leftChild]>heap[rightChild]?leftChild:rightChild;
    }
     
    /**
     *  This method used to print all element of the heap
     *  
     */
    public void printHeap()
        {
            System.out.print("nHeap = ");
            for (int i = 0; i < heapSize; i++)
                System.out.print(heap[i] +" ");
            System.out.println();
        }
    /**
     *  This method returns the max element of the heap.
     *  complexity: O(1)
     */
     public int findMax(){
         if(isEmpty())
             throw new NoSuchElementException("Heap is empty.");
         return heap[0];
     }
      
     public static void main(String[] args){
         BinaryHeap maxHeap = new BinaryHeap(10);
         maxHeap.insert(10);
         maxHeap.insert(4);
         maxHeap.insert(9);
         maxHeap.insert(1);
         maxHeap.insert(7);
         maxHeap.insert(5);
         maxHeap.insert(3);
          
         maxHeap.printHeap();
         maxHeap.delete(5);
         maxHeap.printHeap();
          
     }
}

Example 4: heap sort

// @see https://www.youtube.com/watch?v=H5kAcmGOn4Q

function heapify(list, size, index) {
    let largest = index;
    let left = index * 2 + 1;
    let right = left + 1;
    if (left < size && list[left] > list[largest]) {
        largest = left;
    }
    if (right < size && list[right] > list[largest]) {
        largest = right;
    }
    if (largest !== index) {
        [list[index], list[largest]] = [list[largest], list[index]];
        heapify(list, size, largest);
    }
    return list;
}

function heapsort(list) {
    const size = list.length;
    let index = ~~(size / 2 - 1);
    let last = size - 1;
    while (index >= 0) {
        heapify(list, size, --index);
    }
    while (last >= 0) {
        [list[0], list[last]] = [list[last], list[0]];
        heapify(list, --last, 0);
    }
    return list;
}

heapsort([4, 7, 2, 6, 4, 1, 8, 3]);

Example 5: heapify down

Heapify down is used when we remove the top element from a heap. Removal of an element is done by swapping the top element with the last element at the bottom of the tree, removing the last element, and then heapfying the new top element down to maintain the heap property. Because this moves down the heap tree, it must perform two comparisons per iteration, with the left child and the right child elements, then swap with the smaller one. Because of this, heapify down is usually more complex to implement than heapify up.