Heaps

2 A complete binary tree Nodes contain Comparable objects Each node contains no smaller (or no larger) than objects in its descendants Maxheap Object in a node is ≥ its descendant objects Minheap Object in a node is ≤ descendant objects Note contrast of uses of the word "heap" The ADT heap The heap from which memory is allocated (by the OS) when new executes

3 Heaps (a) maxheap and (b) minheap containing the same values Consider how to use a heap to implement a priority queue…

4 The ADT Heap Interface used for implementation of maxheap public interface MaxHeapInterface { public void add(Comparable newEntry); public Comparable removeMax(); public Comparable getMax(); public boolean isEmpty(); public int getSize(); public void clear(); } // end MaxHeapInterface

5 Using an Array to Represent a Heap (a) A complete binary tree with its nodes numbered in level order; (b) its representation as an array.

6 Using an Array to Represent a Heap When a binary tree is complete Can use level-order traversal to store data in consecutive locations of an array Enables easy location of the data in a node's parent or children Parent of a node at i is found at i/2 (unless i is 1) Children of node at i found at indices 2i and 2i + 1

7 public class MaxHeap implements MaxHeapInterface, java.io.Serializable { private Comparable[ ] heap; // array of heap entries private static final int DEFAULT_MAX_SIZE = 25; private int lastIndex;// index of last entry //****************************************************************** public MaxHeap( ) { heap = new Comparable[DEFAULT_MAX_SIZE]; lastIndex = 0; } //****************************************************************** public MaxHeap(int maxSize) { heap = new Comparable[maxSize]; lastIndex = 0; } //****************************************************************** public Comparable getMax( ) { Comparable root = null; if (!isEmpty()) root = heap[1]; return root; } //****************************************************************** public boolean isEmpty( ) { return lastIndex < 1; } //****************************************************************** public int getSize( ) { return lastIndex; } //****************************************************************** public void clear( ) { for (; lastIndex > -1; lastIndex--) heap[lastIndex] = null; lastIndex = 0; } //******************************************************************... Beginning class MaxHeap

8 Adding an Entry The steps in adding 85 to a maxheap

9 Adding an Entry Begin at next available position for a leaf Follow path from this leaf toward root until correct position for new entry is found As this is done Move entries from parent to child Makes room for new entry

10 Adding an Entry Revision of the previous example that avoids swaps.

11 Adding an Entry Array representation of steps in the example

12 Adding an Entry Array representation of steps in the example (continued)

13 Adding an Entry Algorithm for adding new entry to a heap public void add(Comparable newEntry) { lastIndex++; if (lastIndex >= heap.length) doubleArray(); // expand array int newIndex = lastIndex; int parentIndex = newIndex/2; while ( (newIndex > 1) && newEntry.compareTo(heap[parentIndex]) > 0) { heap[newIndex] = heap[parentIndex]; newIndex = parentIndex; parentIndex = newIndex/2; } heap[newIndex] = newEntry; }

14 Removing the Root Steps to remove the entry in root of the previous maxheap

15 Removing the Root Steps that transform a semiheap (i.e., a heap, except for the root) into a heap without swaps.

16 Removing the Root To remove a heap's root Replace the root with heap's last child This forms a semiheap Then use the method reheap Transforms the semiheap to a heap

17 private void reheap(int rootIndex) { boolean done = false; Comparable orphan = heap[rootIndex]; int largerChildIndex = 2*rootIndex; // index of left child, if any while (!done && (largerChildIndex <= lastIndex) ) { int rightChildIndex = largerChildIndex + 1; if ( (rightChildIndex <= lastIndex) && heap[rightChildIndex].compareTo(heap[largerChildIndex]) > 0) { largerChildIndex = rightChildIndex; } if (orphan.compareTo(heap[largerChildIndex]) < 0) { heap[rootIndex] = heap[largerChildIndex]; rootIndex = largerChildIndex; largerChildIndex = 2*rootIndex; // index of next left child } else done = true; } heap[rootIndex] = orphan; }

18 public Comparable removeMax() { Comparable root = null; if (!isEmpty()) { root = heap[1]; // return value heap[1] = heap[lastIndex]; // form a semiheap lastIndex--; // decrease size reheap(1); // transform to a heap } // end if return root; } // end removeMax

19 Creating a Heap The steps in adding 20, 40, 30, 10, 90, and 70 to a heap using add().

20 Creating a Heap The steps in creating a heap by using reheap. More efficient to use reheap than to use add

21 public MaxHeap(Comparable[] entries) { lastIndex = entries.length; heap = new Comparable[lastIndex + 1]; // copy given array to data field for (int index = 0; index < entries.length; index++) heap[index+1] = entries[index]; // create heap for (int index = heap.length/2; index > 0; index--) reheap(index); } Another Constructor for Class MaxHeap Note: entries in array start at 0; entries in heap start at 1

22 Heapsort Possible to use a heap to sort an array Place array items into a maxheap Then remove them Items will be in descending order Place them back into the original array and they will be in order (but requires additional storage)

23 Heapsort A trace of heapsort (first steps…)

24 Heapsort A trace of heapsort (continued)

25 Heapsort A trace of heapsort (continued)

26 Heapsort A trace of heapsort (continued)

27 Heapsort Implementation of heapsort public static void heapSort(Comparable[] array, int n) { // create first heap for (int rootIndex = n/2-1; rootIndex >= 0; rootIndex--) reheap(array, rootIndex, n-1); swap(array, 0, n-1); for (int lastIndex = n-2; lastIndex > 0; lastIndex--) { reheap(array, 0, lastIndex); swap(array, 0, lastIndex); } } private static void reheap(Comparable[] heap, int rootIndex, int lastIndex) { // Similar to statements from previous code. See next page…... } What is the efficiency?

28 Heapsort Implementation of heapsort public static void heapSort(Comparable[] array, int n) { // create first heap for (int rootIndex = n/2-1; rootIndex >= 0; rootIndex--) reheap(array, rootIndex, n-1); swap(array, 0, n-1); for (int lastIndex = n-2; lastIndex > 0; lastIndex--) { reheap(array, 0, lastIndex); swap(array, 0, lastIndex); } } private static void reheap(Comparable[] heap, int rootIndex, int lastIndex) { // Similar to statements from previous code. See next page…... } Efficiency is O(n log n). However, quicksort is usually a better choice. Efficiency is O(n log n). However, quicksort is usually a better choice.

29 private void reheap(reheap(Comparable[] heap, int rootIndex, int lastIndex) { boolean done = false; Comparable orphan = heap[rootIndex]; int largerChildIndex = 2 * rootIndex + 1; // index of left child, if any while (!done && (largerChildIndex <= lastIndex) ) { int rightChildIndex = largerChildIndex + 1; if ( (rightChildIndex <= lastIndex) && heap[rightChildIndex].compareTo(heap[largerChildIndex]) > 0) { largerChildIndex = rightChildIndex; } if (orphan.compareTo(heap[largerChildIndex]) < 0) { heap[rootIndex] = heap[largerChildIndex]; rootIndex = largerChildIndex; largerChildIndex = 2 * rootIndex + 1; // index of next left child } else done = true; } heap[rootIndex] = orphan; }