© 2004 Goodrich, Tamassia Priority Queues1 Heaps: Tree-based Implementation of a Priority Queue.

Priority Queues 1© 2004 Goodrich, Tamassia

Heaps: Tree-based Implementation of a Priority Queue


Priority Queue ADT (§ 7.1.3)

A priority queue stores a collection of entriesEach entry is a pair(key, value)Main methods of the Priority Queue ADT

insert(k, x)inserts an entry with key k and value x

removeMin()removes and returns the entry with smallest key

Additional methods min()

returns, but does not remove, an entry with smallest key

size(), isEmpty()

Applications: Standby flyers Auctions Stock market


Implementing Priority Queue with Linked Lists

Implementation with an unsorted list

Performance: insert takes O(1) time

since we can insert the item at the beginning or end of the sequence

removeMin and min take O(n) time since we have to traverse the entire sequence to find the smallest key

Implementation with a sorted list

Performance: insert takes O(n) time

since we have to find the place where to insert the item

removeMin and min take O(1) time, since the smallest key is at the beginning

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Can we do better?Yes, using Heaps ,which are built using Trees :

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What is a TreeIn computer science, a tree is an abstract model of a hierarchical structureA tree consists of nodes with a parent-child relationApplications:

Organization charts File systems Programming

environments

Computers”R”Us

Sales R&DManufacturing

Laptops DesktopsUS International

Europe Asia Canada

Priority Queues 6© 2004 Goodrich, Tamassia subtree

Tree TerminologyRoot: node without parent (A)Internal node: node with at least one child (A, B, C, F)External node (a.k.a. leaf ): node without children (E, I, J, K, G, H, D)Ancestors of a node: parent, grandparent, grand-grandparent, etc.Depth of a node: number of ancestorsHeight of a tree: maximum depth of any node (3)Descendant of a node: child, grandchild, grand-grandchild, etc.

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Subtree: tree consisting of a node and its descendants


Binary TreesA binary tree is a tree with the following properties:

Each internal node has at most two children (exactly two for proper binary trees)

The children of a node are an ordered pair

We call the children of an internal node left child and right childAlternative recursive definition: a binary tree is either

a tree consisting of a single node, or

a tree whose root has an ordered pair of children, each of which is a binary tree

Applications: arithmetic

expressions decision processes TODAY: HEAPS!

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(Binary) Tree ADTWe use positions to abstract nodesGeneric methods:

int size() boolean isEmpty() Iterator elements() Iterator<Position>

positions()

Accessor methods: Position root() Position parent(p) Iterator<Position>

children(p)

Query methods: boolean isInternal(p) boolean isExternal(p) boolean isRoot(p)

Update method: object replace (p, o)

Additional methods for BinaryTree (extends Tree):

position left(p) position right(p) boolean hasLeft(p) boolean hasRight(p)


Linked Structure for Binary Trees

A node is represented by an object storing

Element Parent node Left child node Right child node

Node objects implement the Position ADT

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Heaps

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Recall Priority Queue ADT:

A priority queue stores a collection of entriesEach entry is a pair(key, value)Main methods of the Priority Queue ADT

insert(k, v)inserts an entry with key k and value v

removeMin()removes and returns the entry with smallest key

Additional methods min()

returns, but does not remove, an entry with smallest key

size(), isEmpty()


HeapsA heap is a binary tree storing keys at its nodes and satisfying the following two properties:

1. Heap-Order: for every internal node v other than the root,key(v) key(parent(v))

2. Complete Binary Tree: let h be the height of the heap for i 0, … , h 1, there are

2i nodes of depth i at depth h 1, the internal

nodes are to the left of the external nodes

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The last node of a heap is the rightmost node of depth h

last node


Height of a HeapTheorem: A heap storing n keys has height O(log n)

Proof: (we apply the complete binary tree property) Let h be the height of a heap storing n keys Since there are 2i keys at depth i 0, … , h 1 and at least

one key at depth h, we have n 1 2 4 … 2h1 1

Thus, n 2h , i.e., h log n

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Heap Implementation of a Priority Queue

We store a (key, value) item at each internal node

We keep track of the position of the last node

(2, Sue)

(6, Mark)

(5, Pat)

(9, Jeff) (7, Anna)

For simplicity, in later pictures we show only the keys, instead of the (key, value) pairs


Insertion into a Heap

Method insert(k,v) of the priority queue corresponds to the insertion of a key k to the heapThe insertion algorithm consists of three steps

Find the insertion node z (the new last node)

Store k at z Restore the heap-order

property (discussed next)

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insertion node

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UpheapAfter the insertion of a new key k, the heap-order property may be violatedAlgorithm upheap restores the heap-order property by swapping k along an upward path from the insertion nodeUpheap terminates when the key k reaches the root or a node whose parent has a key smaller than or equal to k Since a heap has height O(log n), upheap runs in O(log n) time

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Removal from a HeapMethod removeMin of the priority queue ADT corresponds to the removal of the root key from the heapThe removal algorithm consists of three steps

Replace the root key with the key of the last node w

Remove w Restore the heap-order

property (discussed next)

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last node

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new last node


DownheapAfter replacing the root key with the key k of the last node, the heap-order property may be violatedAlgorithm downheap restores the heap-order property by swapping key k along a downward path from the rootUpheap terminates when key k reaches a leaf or a node whose children have keys greater than or equal to k Since a heap has height O(log n), downheap runs in O(log n) time

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Updating the Last NodeThe insertion node can be found by traversing a path of O(log n) nodes

Go up until a left child or the root is reached If a left child is reached, go to the right child Go down left until a leaf is reached

Similar algorithm for updating the last node after a removal


Extra:Array-Based Implementationof Binary Trees and Heaps


Array-Based Representation of Binary Trees

nodes are stored in an array

…

let rank(node) be defined as follows: rank(root) = 1 if node is the left child of parent(node),

rank(node) = 2*rank(parent(node)) if node is the right child of parent(node),

rank(node) = 2*rank(parent(node))+1

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Array-List based Heap Implementation

We can represent a heap with n keys by means of an array list of length n 1

For the node at rank i the left child is at rank 2i the right child is at rank 2i 1

Links between nodes are not explicitly stored

The cell of at rank 0 is not used

Operation insert corresponds to inserting at rank n 1

Operation removeMin corresponds to removing at rank n

Note that insert at rank n 1 and delete at rank n are O(1)

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Extra:Bottom-Up Heap Construction


We can construct a heap storing n given keys in using a bottom-up construction with log n phasesIn phase i, pairs of heaps with 2i 1 keys are merged into heaps with 2i11 keys

Bottom-up Heap Construction

2i 1 2i 1

2i11


Example

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Example (contd.)

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Example (contd.)

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Example (end)

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AnalysisWe visualize the worst-case time of a downheap with a proxy path that goes first right and then repeatedly goes left until the bottom of the heap (this path may differ from the actual downheap path)Since each node is traversed by at most two proxy paths, the total number of nodes of the proxy paths is O(n) Thus, bottom-up heap construction runs in O(n) time Bottom-up heap construction is faster than n successive insertions and speeds up the first phase of heap-sort


Extra:Application of Priority Queue to Sorting


Priority Queue Sorting (§ 7.1.4)

We can use a priority queue to sort a set of comparable elements

1. Insert the elements one by one with a series of insert operations

2. Remove the elements in sorted order with a series of removeMin operations

The running time of this sorting method depends on the priority queue implementation

Algorithm PQ-Sort(S, C)Input sequence S, comparator C for the elements of SOutput sequence S sorted in increasing order according to CP priority queue with

comparator Cwhile S.isEmpty ()

e S.removeFirst ()P.insert (e, 0)

while P.isEmpty()e

P.removeMin().key()S.insertLast(e)


Selection-Sort

Selection-sort is the variation of PQ-sort where the priority queue is implemented with an unsorted sequenceRunning time of Selection-sort:

1. Inserting the elements into the priority queue with n insert operations takes O(n) time

2. Removing the elements in sorted order from the priority queue with n removeMin operations takes time proportional to

1 2 …n

Selection-sort runs in O(n2) time


Selection-Sort Example Sequence S Priority Queue PInput: (7,4,8,2,5,3,9) ()

Phase 1(a) (4,8,2,5,3,9) (7)(b) (8,2,5,3,9) (7,4).. .. ... . .(g) () (7,4,8,2,5,3,9)

Phase 2(a) (2) (7,4,8,5,3,9)(b) (2,3) (7,4,8,5,9)(c) (2,3,4) (7,8,5,9)(d) (2,3,4,5) (7,8,9)(e) (2,3,4,5,7) (8,9)(f) (2,3,4,5,7,8) (9)(g) (2,3,4,5,7,8,9) ()


Insertion-Sort

Insertion-sort is the variation of PQ-sort where the priority queue is implemented with a sorted sequenceRunning time of Insertion-sort:

1. Inserting the elements into the priority queue with n insert operations takes time proportional to

1 2 …n2. Removing the elements in sorted order from the

priority queue with a series of n removeMin operations takes O(n) time

Insertion-sort runs in O(n2) time


Insertion-Sort ExampleSequence S Priority queue P

Input: (7,4,8,2,5,3,9) ()

Phase 1 (a) (4,8,2,5,3,9) (7)

(b) (8,2,5,3,9) (4,7)(c) (2,5,3,9) (4,7,8)(d) (5,3,9) (2,4,7,8)(e) (3,9) (2,4,5,7,8)(f) (9) (2,3,4,5,7,8)(g) () (2,3,4,5,7,8,9)

Phase 2(a) (2) (3,4,5,7,8,9)(b) (2,3) (4,5,7,8,9).. .. ... . .(g) (2,3,4,5,7,8,9) ()


In-place Insertion-sortInstead of using an external data structure, we can implement selection-sort and insertion-sort in-placeA portion of the input sequence itself serves as the priority queueFor in-place insertion-sort

We keep sorted the initial portion of the sequence

We can use swaps instead of modifying the sequence

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Heap-Sort

Consider a priority queue with n items implemented by means of a heap

the space used is O(n) methods insert and

removeMin take O(log n) time

methods size, isEmpty, and min take time O(1) time

Using a heap-based priority queue, we can sort a sequence of n elements in O(n log n) timeThe resulting algorithm is called heap-sortHeap-sort is much faster than quadratic sorting algorithms, such as insertion-sort and selection-sort

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© 2004 Goodrich, Tamassia Priority Queues1 Heaps: Tree-based Implementation of a Priority Queue.

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