Minimum Spanning Trees

Transcription

1 Minimum Spanning Trees 1

2 Minimum- Spanning Trees 1. Concrete example: computer connection. Definition of a Minimum- Spanning Tree

3 Concrete example Imagine: You wish to connect all the computers in an office building using the least amount of cable - Each vertex in a graph G represents a computer - Each edge represents the amount of cable needed to connect all computers

4 Spanning Tree A spanning tree of G is a subgraph which is tree (acyclic) and connect all the vertices in V. Spanning tree has only V - 1 edges.

5 Problem: Laying Telephone Wire Central office

6 Wiring: Naive Approach Central office Expensiv e!

7 Wiring: Better Approach Central office Minimize the total length of wire connecting the customers

8 Spanning Trees A spanning tree of a graph is just a subgraph that contains all the vertices and is a tree. A graph may have many spanning trees. Graph A Some Spanning Trees from Graph A o r o r o r

9 Complete Graph All 16 of its Spanning Trees

10 Total Number of Spanning Trees A complete graph with n vertices has n (n-) spanning trees.(cayley's formula) is 1 6 is is 100 million 100 is 1016 Compare: there are 1.76*106 seconds in a year. A nanosecond is one billionth (10-) of a second. (An electrical signal can travel about 0cm in a nanosecond.) There are 1.76*101 nanoseconds in a year. We are not going to be able to find all spanning trees for large graphs even on the fastest computers, at least not in our lifetimes. We have to get smart about trees. 10

11 Minimum Spanning Tree Input: Undirected connected graph G = (V, E) and weight function w : E R, Output: A Minimum spanning tree T : tree that connects all the vertices and minimizes Greedy Algorithms Generic MST algorithm Kruskal s algorithm Prim s algorithm w( T) w( u, v) ( u, v) T 11

12 Hallmark for greedy algorithms Greedy-choice property A locally optimal choice is globally optimal. Theorem. Let T be the MST of G = (V, E), and let A V. Suppose that (u, v) E is the least-weight edge connecting A to V A. Then, (u, v) T. 1 1

13 Growing a Minimum Spanning Tree (MST) Generic algorithm Grow MST one edge at a time Manage a set of edges A, maintaining the following loop invariant: Prior to each iteration, A is a subset of some MST At each iteration, we determine an edge (u, v) that can be added to A without violate this invariant A {(u, v)} is also a subset of a MST (u, v) is called a safe edge for A 1 1

14 GENERIC-MST Loop in lines -4 is executed V - 1 times Any MST tree contains V - 1 edges The execution time depends on how to find a safe edge

15 How to Find A Safe Edge? Theorem.1. Let A be a subset of E that is included in some MST, let (S, V-S) be any cut of G that respects A, and let (u, v) be a light edge crossing (S, V-S). Then edge (u, v) is safe for A Cut (S, V-S): a partition of V Crossing edge: one endpoint in S and the other in V-S A cut respects a set of A of edges if no edges in A crosses the cut A light edge crossing a cut if its weight is the minimum of any edge crossing the cut

16 16 16

17 Illustration of Theorem.1 A={(a,b}, (c, i}, (h, g}, {g, f}} S={a, b, c, i, e}; V-S = {h, g, f, d} many kinds of cuts satisfying the requirements of Theorem.1 (c, f) is the light edges crossing S and V-S and will be a safe edge 17

18 Example: MST 1

19 Example: MST 1

20 Kruskal's Algorithm Edge based algorithm Greedy strategy: From the remaining edges, select a least-cost edge that does not result in a cycle when added to the set of already selected edges Repeat V -1 times 0

21 Kruskal's Algorithm INPUT: edge-weighted graph G = (V, E), with V = n OUTPUT: a spanning tree A of G touches all vertices, has n-1 edges of minimum cost ( = total edge weight) Algorithm: Start with A empty, Add the edges one at a time, in increasing weight order An edge is accepted it if connects vertices of distinct trees (if the edge does not form a cycle in A) until A contains n-1 edges 1

22 Kruskal's Algorithm MST-Kruskal(G,w) 1 A for each vertex v V[G] do Make-Set(v) //creates set containing v (for initialization) 4 sort the edges of E for each (u,v)e do 6 if Find-Set(u) Find-Set(v) then // different component 7 A A {(u,v)} Union(Set(u),Set(v)) // merge return A

23 Data Structures For Kruskal s Algorithm Does the addition of an edge (u, v) to T result in a cycle? Each component of T is a tree. When u and v are in the same component, the addition of the edge (u, v) creates a cycle different components, the addition of the edge (u, v) does not create a cycle

24 Data Structures For Kruskal s Algorithm Each component of T is defined by the vertices in the component. Represent each component as a set of vertices. {1,,, 4}, {, 6}, {7, } Two vertices are in the same component iff they are in the same set of vertices

25 Data Structures For Kruskal s Algorithm When an edge (u, v) is added to T, the two components that have vertices u and v combine to become a single component In our set representation of components, the set that has vertex u and the set that has vertex v are united. {1,,, 4} + {, 6} {1,,, 4,, 6}

26 Kruskal s Algorithm ? 1?

27 Kruskal s Algorithm ? ? ?

28 Kruskal s Algorithm ? ? ?

29 Kruskal s Algorithm 1? ? ?

30 A B 4 6 C D 1 E 4 F 0

31 A B 4 6 C D 1 E 4 F 1

32 A B 4 6 C D 1 E 4 F

33 A B 4 6 C D 1 E 4 F

34 A B 4 6 C D 1 E 4 F 4

35 A B 4 6 C D cycle!! 1 E 4 F

36 A B 4 6 C D 1 E 4 F 6

37 A B 4 6 C D 1 E 4 F 7

38 minimum- spanning tree A B C D E 1 F

39 Kruskal's Algorithm MST-Kruskal(G,w) 1 A for each vertex v V[G] do // takes O(V) Make-Set(v) 4 sort the edges of E // takes O(E lg E) // takes O(E) for each (u,v)e, in nondecreasing of weight do 6 if Find-Set(u) Find-Set(v) then 7 A A {(u,v)} Union(Set(u),Set(v)) return A

40 Running Time of Kruskal s Algorithm Kruskal s Algorithm consists of two stages. Initializing the set A in line 1 takes O(1) time. Sorting the edges by weight in line 4. takes O(E lg E) Performing V MakeSet() operations for loop in lines -. E FindSet(), for loop in lines -. V - 1 Union(), for loop in lines -. which takes O(V + E) The total running time is O(E lg E) We have lg E = O(lg V) because # of E = V-1 So total running time becomes O(E lg V). 40

41 Prim s Algorithm The tree starts from an arbitrary root vertex r and grows until the tree spans all the vertices in V. At each step, Adds only edges that are safe for A. When algorithm terminates, edges in A form MST. Vertex based algorithm. Grows one tree T, one vertex at a time 41

42 Prim s Algorithm MST-Prim(G,w,r) //G: graph with weight w and a root vertex r 1 for each u V[G]{ key[u] p[u] NULL // parent of u } 4 key[r] 0 Q = BuildMinHeap(V,key); // Q vertices out of T 6 while Q do 7 u ExtractMin(Q) // making u part of T for each v Adj[u] do if v Q and w(u,v) key[v] then 10 p[v] u 11 key[v] w(u,v) 4 updating keys For each vertex v, key [v] is min weight of any edge connecting v to a vertex in tree. key [v]= if there is no edge and p [v] names parent of v in tree. When algorithm terminates the min-priority queue Q is empty.

43 Prim s Algorithm Lines 1- set the key of each vertex to (except root r, whose key is set to 0 first vertex processed). Also, set parent of each vertex to NULL, and initialize min-priority queue Q to contain all vertices. Line 7 identifies a vertex u є Q Removing u from set Q adds it to set Q-V of vertices in tree, thus adding (u, p[ u]) to A. The for loop of lines -11 update key and p fields of every vertex v adjacent to u but not in tree. 4

44 Run on example graph

45 Run on example graph key[u] = 1 4

46 Run on example graph 6 4 r 10 0 Pick a start vertex r 1 46

47 Run on example graph 6 4 u Red vertices have been removed from Q 47

48 Run on example graph 6 4 u Red arrows indicate parent pointers 4

49 Run on example graph 6 4 u

50 Run on example graph u 1 Extract_min from Q 0

51 Run on example graph u 1 1

52 Run on example graph u 1

53 Run on example graph u 1

54 Run on example graph u 1 4

55 Run on example graph u 1

56 Run on example graph u 1 6

57 Run on example graph u 1 7

58 Run on example graph u 1

59 Run on example graph u 1

60 Run on example graph u 1 60

61 Run on example graph u

62 Run on example graph u 6

63 Run on example graph u 6

64 Prim s Running Time What is the hidden cost in this code? Extract-Min is executed V times MST-Prim(G,w,r) 1 for each u V[Q] Decrease-Key is executed O( E ) times key[u] p[u] NULL while loop is executed V times 4 key[r] 0 Q = BuildHeap(V,key); //Q vertices out of T 6 while Q do 7 u ExtractMin(Q) // making u part of T for each v Adj[u] do if v Q and w(u,v) < key[v] then 10 p[v] u 11 key[v] w(u,v) DecreaseKey(v, w(u,v)); updating keys 64

65 Prim s Running Time Time complexity depends on data structure Q Binary heap: O(E lg V): BuildHeap takes O(log V) time number of while iterations: V ExtractMin takes O(lg V) time total number of for iterations: E DecreaseKey takes O(lg V) time Hence, Time = log V + V.T(ExtractMin) + E.T(DecreaseKey) Time = O(V lg V + E lg V) = O(E lg V) Since E V 1 (because G is connected) 6

66 Minimum bottleneck spanning tree A bottleneck edge is the highest weighted edge in a spanning tree. A spanning tree is a minimum bottleneck spanning tree (or MBST) if the graph does not contain a spanning tree with a smaller bottleneck edge weight. A MST is necessarily a MBST, but a MBST is not necessarily a MST. 66