Disjoint Sets and Advanced Tree Topics HKOI Training 2007 14 Apr 2007 Acknowledgement: Presentation...

Disjoint Sets and Advanced Tree Topics

HKOI Training 200714 Apr 2007

Acknowledgement:Presentation Modified from “Minimum Spanning Trees” by Liu Chi Man (cx), 25 Mar 2006

2

Prerequisites

Asymptotic complexity Set theory Elementary graph theory Priority queues (or heaps)

3

Graphs

A graph is a set of vertices and a set of edges

G = (V, E) Number of vertices = |V| Number of edges = |E| We assume simple graph, so |E| =

O(|V|2)

4

Roadmap

What is a tree? Disjoint sets Minimum spanning trees Various tree topics

5

Trees in graph theory

In graph theory, a tree is an acyclic, connected graph Acyclic means “without cycles”

6

Properties of trees

|E| = |V| - 1 |E| = (|V|)

Between any pair of vertices, there is a unique path

Adding an edge between a pair of non-adjacent vertices creates exactly one cycle

Removing an edge from the tree breaks the tree into two smaller trees

7

Definition

The following four conditions are equivalent: G is connected and acyclic G is connected and |E| = |V| - 1 G is acyclic and |E| = |V| - 1 Between any pair of vertices in G, there

exists a unique path G is a tree if at least one of the

above conditions is satisfied

8

Recall the Terminology

root

siblings

descendents children

ancestors

parent

9

Other properties of trees

Bipartite Planar A tree with at least two vertices has

at least two leaves (vertices of degree 1)

10

Roadmap

What is a tree? Disjoint sets Minimum spanning trees Various tree topics

11

The Union-Find problem

N balls initially, each ball in its own bag Label the balls 1, 2, 3, ..., N

Two kinds of operations: Pick two bags, put all balls in these

bags into a new bag (Union) Given a ball, find the bag containing it

(Find)

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The Union-Find problem

An example with 4 balls Initial: {1}, {2}, {3}, {4} Union {1}, {3} {1, 3}, {2}, {4} Find 3. Answer: {1, 3} Union {4}, {1,3} {1, 3, 4}, {2} Find 2. Answer: {2} Find 1. Answer {1, 3, 4}

13

Disjoint sets

Disjoint-set data structures can be used to solve the union-find problem

Each bag has its own representative ball {1, 3, 4} is represented by ball 3 (for

example) {2} is represented by ball 2

14

Implementation 1: Naive arrays

Bag[x] := representative of the bag containing x

<O(N), O(1)> Union takes O(N) and Find takes O(1)

Slight modifications give <O(U), O(1)> U is the size of the union

Worst case: O(MN) for M operations

15

Implementation 1: Naive arrays

How to union Bag[x] and Bag[y]? Z := Bag[x]

For each ball v in Z do Bag[v] := Bag[y]

Can I update the balls in Bag[y] instead?

Rule: Update the balls in the smaller bag O(MlgN) for M union operations

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Implementation 2: Forest

A forest is a collection of trees Each bag is represented by a rooted

tree, with the root being the representative ball

1

5 3

6

4

2 7

Example: Two bags --- {1, 3, 5} and {2, 4, 6, 7}.

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Implementation 2: Forest

Find(x) Traverse from x up to the root

Union(x, y) Merge the two trees containing x and y

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Implementation 2: Forest

Initial:

Union 1 3:

Union 2 4:

Find 4:

1 3 42

1

3

42

1

3 4

2

1

3 4

2

19

Implementation 2: Forest

Union 1 4:

Find 4:

1

3

4

2

1

3

4

2

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Implementation 2: Forest

How to represent the trees? Leftmost-Child-Right-Sibling (LCRS)?

Too complicated Parent array

Parent[x] := parent of x If x is a tree root, set Parent[x] := x

21

Implementation 2: Forest

The worst case is still O(MN) for M operations What is the worst case?

Improvements Union-by-rank Path compression

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Union-by-rank

We should avoid tall trees Root of the taller tree becomes the

new root when union So, keep track of tree heights

(ranks)

Good Bad

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Path compression

See also the solution for Symbolic Links (HKOI2005 Senior Final)

Find(x): traverse from x up to root Compress the x-to-root path at the

same time

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Path compression

Find(4)

3

5 1

6

4

2 7

3

5 1

6

4

2 7

The root is 3

The root is 3

The root is 3

3

5 164

2 7

25

U-by-rank + Path compression

We ignore the effect of path compression on tree heights to simplify U-by-rank

U-by-rank alone gives O(MlgN) U-by-rank + path compression gives

O(M(N)) : inverse Ackermann function

(N) 5 for practically large N

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Roadmap

What is a tree? Disjoint sets Minimum spanning trees Various tree topics

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Minimum spanning trees

Given a connected graph G = (V, E), a spanning tree of G is a graph T such that T is a subgraph of G T is a tree T contains every vertex of G

A connected graph must have at least one spanning tree (why?)

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Minimum spanning trees

Given a weighted connected graph G, a minimum spanning tree T* of G is a spanning tree of G with minimum total edge weight Is it unique?

Application: Minimizing the total length of wires needed to connect up a collection of computers

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Minimum spanning trees

Two algorithms Kruskal’s algorithm Prim’s algorithm

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Kruskal’s algorithm

Choose edges in ascending weight greedily, while preventing cycles

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Kruskal’s algorithm

Algorithm T is an empty set Sort the edges in G by their weights For (in ascending weight) each edge e

do If T {e} is acyclic then

Add e to T

Return T

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Kruskal’s algorithm

How to detect a cycle? Depth-first search (DFS)

O(V) per check O(VE) overall

Disjoint set Vertices are balls, connected components

are bags

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Kruskal’s algorithm

Algorithm (using disjoint-set) T is an empty set Create bags {1}, {2}, …, {V} Sort the edges in G by their weights For (in ascending weight) each edge e do

Suppose e connects vertices x and y If Find(x) Find(y) then

Add e to T, then Union(Find(x), Find(y))

Return T

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Kruskal’s algorithm

The improved time complexity is O(ElgV)

The bottleneck is sorting

35

Prim’s algorithm

In Kruskal’s algorithm, the MST-in-progress scatters around

Prim’s algorithm grows the MST from a “seed”

Prim’s algorithm iteratively chooses the lightest grow-able edge A grow-able edge connects a grown

vertex and a non-grown vertex

36

Prim’s algorithm

Algorithm Let seed be any vertex, and Grown :=

{seed} Initially T is an empty set Repeat |V|-1 times

Let e=(x,y) be the lightest grow-able edge Add e to T Add x and y to Grown

Return T

37

Prim’s algorithm

How to find the lightest grow-able edge? Check all (grown, non-grown) vertex

pairs Too slow

Each non-grown vertex x keeps a value nearest[x], which is the weight of the lightest edge connecting x to some grown vertex

Nearest[x] = if no such edge

38

Prim’s algorithm

How to use nearest? Grow the vertex (x) with the minimum

nearest-value Which edge? Keep track on it!

Since x has just been grown, we need to update the nearest-values of all non-grown vertices

Only need to consider edges incident to x

39

Prim’s algorithm

Try to program Prim’s algorithm You may find that it’s very similar to

Dijkstra’s algorithm for finding shortest paths! Almost only a one-line difference

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Prim’s algorithm

Per round... Finding minimum nearest-value: O(V) Updating nearest-values: O(V) (Overall

O(E)) Overall: O(V2+E) = O(V2) time Using a binary heap,

O(lgV) per Finding minimum O(lgV) per Updating Overall: O(ElgV) time

41

MST Extensions

Second-best MST We don’t want the best!

Online MST See IOI2003 Path Maintenance

Minimum bottleneck spanning tree The bottleneck of a spanning tree is the

weight of its maximum weight edge An algorithm that runs in O(V+E) exists

42

MST Extensions (NP-Hard)

Minimum Steiner Tree No need to connect all vertices, but at

least a given subset B V Degree-bounded MST

Every vertex of the spanning tree must have degree not greater than a given value K

43

Roadmap

What is a tree? Disjoint sets Minimum spanning trees Various tree topics

44

Various tree topics (List)

Center, eccentricity, radius, diameter

Lowest common ancestor (LCA) Tree isomorphism

Canonical representation Prüfer code Counting spanning trees

45

Supplementary readings

Advanced: Disjoint set forest (Lecture slides) Prim’s algorithm Kruskal’s algorithm Center and diameter

Post-advanced (so-called Beginners): Lowest common ancestor Maximum branching