Fibonacci Heaps Priority Queues - Purdue Universitycs180/Spring2010Web/... · 2010. 4. 26. · Heap...

Fibonacci HeapsPriority Queues

John Ross Wallrabenstein

Updates

• Project 4 Grades Released

• Project 5 Questions?

Review

*This usually refers to the size of the structure

Review

• What is a static data structure?

Review

• What is a static data structure?

• Data Structures that do not change* over the lifetime of their use

Review

• What is a dynamic data structure?

Review

• What is a dynamic data structure?

• Data Structures that change over their lifetime, allowing for efficient updates

Review

• Can you describe the Union/Find algorithm and its applications?

• Can you describe the Union By Rank heuristic?

• Can you describe Path Compression?

Union By Rank/** * Combines the set that contains a with the set that contains b. */ public void union(int a, int b) { Node na = findNode(a); Node nb = findNode(b);

if (na == nb) { return; }

// Link the smaller tree under the larger. if (na.rank > nb.rank) { // Delete nb. nb.child.parent = na.child; na.value = b; } else { // Delete na. na.child.parent = nb.child; nb.value = b;

if (na.rank == nb.rank) { nb.rank++; } } }

Path Compression/** * Finds the set containing a given Node. */ private Node findNode(Node node) { int top = 0;

// Find the child of the root element. while (node.parent.child == null) { stack[top++] = node; node = node.parent; }

// Do path compression on the way back down. Node rootChild = node;

while (top > 0) { node = stack[--top]; node.parent = rootChild; }

return rootChild.parent; }

Today’s Goal

• What is a Queue?

• What is a Priority Queue?

• We will implement a Priority Queue efficiently using a Fibonacci Heap

Overview

• Tree Theory

• Heaps

• Fibonacci Heaps

• Priority Queues

Tree Theory

• A free tree is a connected, acyclic undirected graph

• A graph is connected if every pair of vertices is connected by some path

• A graph is acyclic if it contains no cycles

Free Tree

Rooted Trees

• A rooted tree is a free tree where one vertex is distinguished from the others as the root

Rooted Tree

Terminology

• An ancestor of node x is any node on the path from x to the root

• Similarly, if node y is an ancestor of x, then x is a descendent of y

• A leaf node has no descendants

Binary Tree

• A binary tree satisfies the property that every node has at most two children

Overview

• Tree Theory

• Heaps

• Fibonacci Heaps

• Priority Queues

• A heap is a complete binary tree satisfying one of the following heap properties:

• Max-Heap: the parent of every node has a rank greater than or equal to all of its children

• Min-Heap: the parent of every node has a rank less than or equal to all of its children

• Which data structure do you use to organize your priorities?

• e.g. If you use a max-heap, you visit cracked.com before doing CS 180 work

• Assumption: Your last priority has a “higher” rank than your #1 priority

Max-Heap

Maintaining the Heap Property

• We will assume for now that the tree is self-balancing

• e.g. The height is kept at a minimum of log n

• When a node is inserted, the heap may no longer satisfy the {min,max} property

• To “fix” the heap, we recursively heapify so that the {min,max}-property is maintained

• Yes, heapify is the actual term!

• Max-Heapify Invariant:

• We will assume the binary trees rooted at Left(i) and Right(i) are max-heaps

• We will assume the inserted node x may be smaller than its children

• This violates the property

• Basic Idea:

• Let node i “float down” to its proper position so that the max-heap property is satisfied

Max-Heapify

Overview

• Tree Theory

• Heaps

• Fibonacci Heaps

• Priority Queues

Fibonacci Heaps

• A Fibonacci Heap is a forest of min-heap trees

• Their name is derived from the Fibonacci Numbers, which are used in their asymptotic analysis

Operations

• We will only concern ourselves with the following operations:

• Insert

• Extract-Min

• There are others, but the additional complexity is not necessary for queues*

*You should thank me for not covering this

Motivation

• Fibonacci Heaps have fast amortized cost relative to other data structures

• Heaps in general support priority queues

Motivation

• While Fibonacci Heaps are min-heaps, we can easily use them as priority queues

• How?

• Define lesser values to have higher priority (this is somewhat natural)

Underlying Structure

• Fibonacci Heaps are commonly implemented using a circular doubly-linked list

• Brief review of standard operations over a doubly-linked list

• See board

WARNING

• The following image has been found to induce disgust, confusion and seizures

• I promise things won’t seem as bad as they look*!

*By not covering your eyes, you absolve me of all legal liability with respect to your mental/emotional well-being

C-DLL Structure

*Ignore the dark, or “marked” nodes

Fibonacci Heaps

Lecture slides adapted from:

Chapter 19

Kevin Wayne’s slides from the 2007 Princeton University Algorithms y y gclass

Priority Queues Performance

make-heap

Operation

BinaryHeap

LinkedList

FibonacciHeap †

insert

delete min

is-empty 11

delete-min

decrease-key

delete

find-min

† amortizedn = number of elements in priority queue † amortizedn = number of elements in priority queue

Hopeless challenge. O(1) insert, delete- min and decrease- key. Why?

Fibonacci Heaps: Structure

Fibonacci heap.Set of heap- ordered trees.Maintain pointer to minimum element.Set of marked nodes.

find- min takes O(1) time

17 72324 3

4118 52

Heap H

Fibonacci Heaps: Structure

Fibonacci heap.Set of heap- ordered trees.Maintain pointer to minimum element.Set of marked nodes.

roots heap-ordered tree

72317 24 3

26 464118 52

Heap H

Fibonacci Heaps: Notation

Notation

n = number of nodes in heap.

rank(x) = number of children of node x.

rank(H) = max rank of any node in heap H.

trees(H) = number of trees in heap H.

marks(H) = number of marked nodes in heap H.

(Note: terms different in text)

k i( ) k (H) 3 14

72317 24 3

rank = 3 mintrees(H) = 5 marks(H) = 3 n = 14

4118 52

3539 44

Heap H marked

Insert

Fibonacci Heaps: Insert

Insert.

Create a new singleton tree.

Add to root list; update min pointer (if necessary).

insert 21

72317 24 3

4118 52

3539 44

Heap H

Fibonacci Heaps: Insert

Insert.

Create a new singleton tree.

Add to root list; update min pointer (if necessary).

insert 21

723 317 24 21

44Heap H

Fibonacci Heaps: Insert Analysis

Actual cost. O(1)

Change in potential. + 1

(H) = trees(H) + 2 marks(H)

potential of heap H

Amortized cost. O(1)

7 317 24 2123

24 2123

44Heap H

Delete Min

Linking Operation

Linking operation. Make larger root a child of smaller root.

smaller rootlarger root still heap-ordered

4118 5256 24 4118 5215

39 4477

tree T1 tree T2

tree T'

Fibonacci Heaps: Delete Min

Delete minDelete min.Delete minMeld its children into the root list and update min.Consolidate trees so that no two roots have same rankConsolidate trees so that no two roots have same rank.

317237 24

4118 52

39 4435

Delete min.Delete minMeld its children into the root list and update min.Consolidate trees so that no two roots have same rank.

411723 18 527 24

3930 26 46 44

Delete min.Delete minMeld its children into the root list and update min.Consolidate trees so that no two roots have same rank:– Repeatedly link two trees of the same rank until no pair

of roots has the same rank (= degree)

mincurrent

411723 18 527 24

26 46 44

Delete minDelete min.

Delete min. Meld its children into the root list and update min.

Consolidate trees so that no two roots have same rank.

0 1 2 3

currentmin

411723 18 527 24

26 46 44

Delete min.

0 1 2 3

mincurrent

411723 18 527 24

26 46 44

Delete min.

0 1 2 3

411723 18 527 24

26 46 44current

Delete min.

Delete min

Meld its children into the root list and update min.

0 1 2 3

411723 18 527 24

26 46 44current

link 23 into 17

Delete min.

0 1 2 3

4117 18 527 24

392330

26 46 44current

link 17 into 7

Delete min.

0 1 2 3

current

417 18 5224

393017

26 46 44

link 24 into 7

Delete min.

Delete min. Meld its children into root list; update min.

0 1 2 3

current

417 18 52

Delete min.

Delete min; meld its children into root list; update min.

0 1 2 3

current

417 18 52

Delete min.

0 1 2 3

current

417 18 52

Delete min.

0 1 2 3

current

417 18 52

link 41 into 18

Delete min.

0 1 2 3

current

7 1852

394130

26 46 44

Delete min.

0 1 2 3

current

24 3941

26 46 44

Delete min.

301724 3941

26 46 44

Before delete - min

317237 24

4118 52

minAfter consolidation, no7

1724 3941

18After consolidation, no two roots have the same degree and the largest degree in O(log n)

24 3941

35After delete - min

Fibonacci Heaps Priority Queues - Purdue Universitycs180/Spring2010Web/... · 2010. 4. 26. · Heap...

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