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Chapter 13: Graphs I Objectives: Graph ADT: Operations Graph Implementation: Data structures Graph Traversals: DFS and BFS Directed graphs Weighted graphs Shortest paths Minimum Spanning Trees (MST) CSC311: Data Structures 1 Graphs A graph is a pair (V, E), where – V is a set of nodes, called vertices – E is a collection of pairs of vertices, called edges – Vertices and edges are positions and store elements Example: – A vertex represents an airport and stores the three-letter airport code – An edge represents a flight route between two airports and stores the mileage of the route SFO PVD ORD LGA HNL Graphs I LAX DFW CSC311: Data Structures MIA 2 Edge Types Directed edge – ordered pair of vertices (u,v) – first vertex u is the origin – second vertex v is the destination – e.g., a flight ORD flight AA 1206 PVD ORD 849 miles PVD Undirected edge – unordered pair of vertices (u,v) – e.g., a flight route Directed graph – all the edges are directed – e.g., route network Undirected graph – all the edges are undirected – e.g., flight network Graphs I CSC311: Data Structures 3 Applications Electronic circuits cslab1a cslab1b – Printed circuit board – Integrated circuit Transportation networks math.brown.edu cs.brown.edu – Highway network – Flight network brown.edu Computer networks – Local area network – Internet – Web qwest.net att.net cox.net Databases John – Entity-relationship diagramPaul Graphs I CSC311: Data Structures David 4 Terminology End vertices (or endpoints) of an edge – U and V are the endpoints of a a Edges incident on a vertex – a, d, and b are incident on V Adjacent vertices – U and V are adjacent U V b d X c Degree of a vertex – X has degree 5 e W – h and i are parallel edges j Z i g f Parallel edges h Y Self-loop – j is a self-loop Graphs I CSC311: Data Structures 5 Terminology (cont.) Path – sequence of alternating vertices and edges – begins with a vertex – ends with a vertex – each edge is preceded and followed by its endpoints Simple path a U – path such that all its vertices and edges are distinct Examples – P1=(V,b,X,h,Z) is a simple path – P2=(U,c,W,e,X,g,Y,f,W,d,V) is a path that is not simple Graphs I CSC311: Data Structures c V b d P2 P1 X e W h Z g f Y 6 Terminology (cont.) Cycle – circular sequence of alternating vertices and edges – each edge is preceded and followed by its endpoints Simple cycle – cycle such that all its vertices and edges are distinct Examples a U c – C1=(V,b,X,g,Y,f,W,c,U,a,) is a simple cycle – C2=(U,c,W,e,X,g,Y,f,W,d,V,a,) is a cycle that is not simple Graphs I CSC311: Data Structures V b d C2 X e C1 g W f h Z Y 7 Properties Property 1 Notation Sv deg(v) = 2m n Proof: each edge is counted twice Property 2 In an undirected graph with no self-loops and no multiple edges m n (n - 1)/2 Proof: each vertex has degree at most (n - 1) m deg(v) number of vertices number of edges degree of vertex vExample – n=4 – m=6 – deg(v) = 3 What is the bound for a directed graph? Graphs I CSC311: Data Structures 8 Main Methods of the Graph ADT Vertices and edges Update methods – are positions – store elements Accessor methods – endVertices(e): an array of the two endvertices of e – opposite(v, e): the vertex opposite of v on e – areAdjacent(v, w): true iff v and w are adjacent – replace(v, x): replace element at vertex v with x – replace(e, x): replace element at edge e with x Graphs I – insertVertex(o): insert a vertex storing element o – insertEdge(v, w, o): insert an edge (v,w) storing element o – removeVertex(v): remove vertex v (and its incident edges) – removeEdge(e): remove edge e Iterator methods – incidentEdges(v): edges incident to v – vertices(): all vertices in the graph – edges(): all edges in the graph CSC311: Data Structures 9 Edge List Structure u Vertex object a – element – reference to position in vertex sequence v c b d w z Edge object – – – – element origin vertex object destination vertex object reference to position in edge sequence u z w v Vertex sequence – sequence of vertex objects a b c d Edge sequence – sequence of edge objects Graphs I CSC311: Data Structures 10 Adjacency List Structure a Edge list structure Incidence sequence for each vertex – sequence of references to edge objects of incident edges v b u u w v w Augmented edge objects – references to associated positions in incidence sequences of end vertices Graphs I a CSC311: Data Structures b 11 Adjacency Matrix Structure a Edge list structure Augmented vertex objects v b u w – Integer key (index) associated with vertex 2D-array adjacency array 0 u 1 – Reference to edge object for adjacent vertices – Null for non nonadjacent vertices 0 0 The “old fashioned” version just has 0 for no edge and 1 for edge Graphs I CSC311: Data Structures 2 1 w 2 1 a 2 v b 12 Asymptotic Performance n vertices, m edges no parallel edges no self-loops Bounds are “big-Oh” Edge List Adjacency List Adjacenc y Matrix Space n+m n+m n2 incidentEdges(v) areAdjacent (v, w) insertVertex(o) m m 1 deg(v) min(deg(v), deg(w)) 1 n 1 n2 insertEdge(v, w, o) 1 1 1 removeVertex(v) m deg(v) removeEdge(e) 1 1 n2 1 Graphs I CSC311: Data Structures 13 Subgraphs A subgraph S of a graph G is a graph such that – The vertices of S are a subset of the vertices of G – The edges of S are a subset of the edges of G A spanning subgraph of G is a subgraph that contains all the vertices of G Graphs I CSC311: Data Structures Subgraph Spanning subgraph 14 Connectivity A graph is connected if there is a path between every pair of vertices A connected component of a graph G is a maximal connected subgraph of G Graphs I Connected graph Non connected graph with two connected components CSC311: Data Structures 15 Trees and Forests A (free) tree is an undirected graph T such that – T is connected – T has no cycles This definition of tree is different from the one of a rooted tree A forest is an undirected graph without cycles The connected components of a forest are trees Graphs I CSC311: Data Structures Tree Forest 16 Spanning Trees and Forests A spanning tree of a connected graph is a spanning subgraph that is a tree A spanning tree is not unique unless the graph is a tree Spanning trees have applications to the design of communication networks A spanning forest of a graph is a spanning subgraph that is a forest Graph Spanning tree Graphs I CSC311: Data Structures 17 Depth-First Search Depth-first search (DFS) is a general technique for traversing a graph A DFS traversal of a graph G – Visits all the vertices and edges of G – Determines whether G is connected – Computes the connected components of G – Computes a spanning forest of G Graphs I DFS on a graph with n vertices and m edges takes O(n + m ) time DFS can be further extended to solve other graph problems – Find and report a path between two given vertices – Find a cycle in the graph Depth-first search is to graphs while Euler tour is to binary trees CSC311: Data Structures 18 DFS Algorithm The algorithm uses a mechanism for setting and getting “labels” of vertices and edges Algorithm DFS(G) Input graph G Output labeling of the edges of G as discovery edges and back edges for all u G.vertices() setLabel(u, UNEXPLORED) for all e G.edges() setLabel(e, UNEXPLORED) for all v G.vertices() if getLabel(v) = UNEXPLORED DFS(G, v) Graphs I Algorithm DFS(G, v) Input graph G and a start vertex v of G Output labeling of the edges of G in the connected component of v as discovery edges and back edges setLabel(v, VISITED) for all e G.incidentEdges(v) if getLabel(e) = UNEXPLORED w opposite(v,e) if getLabel(w) = UNEXPLORED setLabel(e, DISCOVERY) DFS(G, w) else setLabel(e, BACK) CSC311: Data Structures 19 Example unexplored vertex visited vertex unexplored edge discovery edge back edge A A A B D E A D E C Graphs I E C A B D B C CSC311: Data Structures 20 Example (cont.) A B A D E C C A A B D E C Graphs I B B D E D E C CSC311: Data Structures 21 Properties of DFS Property 1 DFS(G, v) visits all the vertices and edges in the connected component of v Property 2 The discovery edges labeled by DFS(G, v) form a spanning tree of the connected component of v Graphs I A B CSC311: Data Structures D E C 22 Analysis of DFS Setting/getting a vertex/edge label takes O(1) time Each vertex is labeled twice – once as UNEXPLORED – once as VISITED Each edge is labeled twice – once as UNEXPLORED – once as DISCOVERY or BACK Method incidentEdges is called once for each vertex DFS runs in O(n + m) time provided the graph is represented by the adjacency list structure – Recall that Graphs I Sv deg(v) = 2m CSC311: Data Structures 23 Path Finding We can specialize the DFS algorithm to find a path between two given vertices u and z using the template method pattern We call DFS(G, u) with u as the start vertex We use a stack S to keep track of the path between the start vertex and the current vertex As soon as destination vertex z is encountered, we return the path as the contents of the stack Graphs I Algorithm pathDFS(G, v, z) setLabel(v, VISITED) S.push(v) if v = z return S.elements() for all e G.incidentEdges(v) if getLabel(e) = UNEXPLORED w opposite(v,e) if getLabel(w) = UNEXPLORED setLabel(e, DISCOVERY) S.push(e) pathDFS(G, w, z) S.pop(e) else setLabel(e, BACK) S.pop(v) CSC311: Data Structures 24 Cycle Finding We can specialize the DFS algorithm to find a simple cycle using the template method pattern We use a stack S to keep track of the path between the start vertex and the current vertex As soon as a back edge (v, w) is encountered, we return the cycle as the portion of the stack from the top to vertex w Graphs I Algorithm cycleDFS(G, v, z) setLabel(v, VISITED) S.push(v) for all e G.incidentEdges(v) if getLabel(e) = UNEXPLORED w opposite(v,e) S.push(e) if getLabel(w) = UNEXPLORED setLabel(e, DISCOVERY) cycleDFS(G, w, z) S.pop(e) else T new empty stack repeat o S.pop() T.push(o) until o = w return T.elements() S.pop(v) CSC311: Data Structures 25 Breadth-First Search Breadth-first search (BFS) is a general technique for traversing a graph A BFS traversal of a graph G – Visits all the vertices and edges of G – Determines whether G is connected – Computes the connected components of G – Computes a spanning forest of G Graphs I BFS on a graph with n vertices and m edges takes O(n + m ) time BFS can be further extended to solve other graph problems – Find and report a path with the minimum number of edges between two given vertices – Find a simple cycle, if there is one CSC311: Data Structures 26 BFS Algorithm The algorithm uses a mechanism for setting and getting “labels” of vertices and edges Algorithm BFS(G) Input graph G Output labeling of the edges and partition of the vertices of G for all u G.vertices() setLabel(u, UNEXPLORED) for all e G.edges() setLabel(e, UNEXPLORED) for all v G.vertices() if getLabel(v) = UNEXPLORED BFS(G, v) Graphs I Algorithm BFS(G, s) L0 new empty sequence L0.insertLast(s) setLabel(s, VISITED) i0 while Li.isEmpty() Li +1 new empty sequence for all v Li.elements() for all e G.incidentEdges(v) if getLabel(e) = UNEXPLORED w opposite(v,e) if getLabel(w) = UNEXPLORED setLabel(e, DISCOVERY) setLabel(w, VISITED) Li +1.insertLast(w) else setLabel(e, CROSS) i i +1 CSC311: Data Structures 27 Example L0 unexplored vertex visited vertex unexplored edge discovery edge cross edge A A L0 L1 B B L0 C D C E A E Graphs I L1 A L1 F F A B C E CSC311: Data Structures D D F 28 Example (cont.) L0 L1 L0 A B C E L0 L1 Graphs I F L0 C E B L2 A B L2 D L1 D L1 CSC311: Data Structures C E D F A B L2 F A C E D F 29 Example (cont.) L0 L1 L0 L1 C E D L1 B L2 F A C E D F A B L2 Graphs I A B L2 L0 C E D F CSC311: Data Structures 30 Properties Notation A Gs: connected component of s Property 1 B BFS(G, s) visits all the vertices and edges of Gs Property 2 The discovery edges labeled by BFS(G, s) form a spanning tree Ts of Gs Property 3 For each vertex v in Li L1 – The path of Ts from s to v has i edges – Every path from s to v in Gs has at least i edges Graphs I CSC311: Data Structures C E L0 F A B L2 D C E D F 31 Analysis Setting/getting a vertex/edge label takes O(1) time Each vertex is labeled twice – once as UNEXPLORED – once as VISITED Each edge is labeled twice – once as UNEXPLORED – once as DISCOVERY or CROSS Each vertex is inserted once into a sequence Li Method incidentEdges is called once for each vertex BFS runs in O(n + m) time provided the graph is represented by the adjacency list structure – Recall that Graphs I Sv deg(v) = 2m CSC311: Data Structures 32 Applications Using the template method pattern, we can specialize the BFS traversal of a graph G to solve the following problems in O(n + m) time – Compute the connected components of G – Compute a spanning forest of G – Find a simple cycle in G, or report that G is a forest – Given two vertices of G, find a path in G between them with the minimum number of edges, or report that no such path exists Graphs I CSC311: Data Structures 33 DFS vs. BFS Applications DFS BFS Spanning forest, connected components, paths, cycles Shortest paths Biconnected components L0 A B C E D L1 DFS Graphs I B L2 F A C E D F BFS CSC311: Data Structures 34 DFS vs. BFS (cont.) Back edge (v,w) Cross edge (v,w) – w is an ancestor of v in the tree of discovery edges – w is in the same level as v or in the next level in the tree of discovery edges L0 A B C E D L1 DFS Graphs I B L2 F A C E D F BFS CSC311: Data Structures 35 Digraphs A digraph is a graph whose edges are all directed E D – Short for “directed graph” C Applications – one-way streets – flights – task scheduling Graphs I B A CSC311: Data Structures 36 Digraph Properties E A graph G=(V,E) such that – Each edge goes in one direction: Edge (a,b) goes from a to b, but not b to a. If G is simple, m < n*(n-1). If we keep in-edges and outedges in separate adjacency lists, we can perform listing of in-edges and out-edges in time proportional to their size. Graphs I CSC311: Data Structures D C B A 37 Digraph Application Scheduling: edge (a,b) means task a must be completed before b can be started ics21 ics22 ics23 ics51 ics53 ics52 ics161 ics131 ics141 ics121 The good life ics151 Graphs I ics171 CSC311: Data Structures 38 Directed DFS We can specialize the traversal algorithms (DFS and BFS) to digraphs by traversing edges only along their direction In the directed DFS algorithm, we have four types of edges – – – – discovery edges back edges forward edges cross edges D C B A directed DFS starting a vertex s determines the vertices reachable from s Graphs I E CSC311: Data Structures A 39 Reachability DFS tree rooted at v: vertices reachable from v via directed paths E E D C D A C A F E B C A Graphs I D CSC311: Data Structures F B 40 Strong Connectivity Each vertex can reach all other vertices a g c d e b f Graphs I CSC311: Data Structures 41 Strong Connectivity Algorithm Pick a vertex v in G. Perform a DFS from v in G. a G: – If there’s a w not visited, print “no”. d Let G’ be G with edges reversed. Perform a DFS from v in G’. – If there’s a w not visited, print “no”. – Else, print “yes”. Running time: O(n+m). Graphs I CSC311: Data Structures g c e b f a G’: g c d f e b 42 Strongly Connected Components Maximal subgraphs such that each vertex can reach all other vertices in the subgraph Can also be done in O(n+m) time using DFS, but is more complicated (similar to biconnectivity). a c d {a,c,g} b {f,d,e,b} e f Graphs I g CSC311: Data Structures 43 Transitive Closure Given a digraph G, the transitive closure of G is the digraph G* such that – G* has the same vertices as G – if G has a directed path from u to v (u v), G* has a directed edge from u to v The transitive closure provides reachability information about a digraph Graphs I CSC311: Data Structures D E B C G A D E B C A G* 44 Computing the Transitive Closure We can perform DFS starting at each vertex – O(n(n+m)) If there's a way to get from A to B and from B to C, then there's a way to get from A to C. Graphs I Alternatively ... Use dynamic programming: The Floyd-Warshall Algorithm CSC311: Data Structures 45 Floyd-Warshall Transitive Closure Idea #1: Number the vertices 1, 2, …, n. Idea #2: Consider paths that use only vertices numbered 1, 2, …, k, as intermediate vertices: i Uses only vertices numbered 1,…,k (add this edge if it’s not already in) j Uses only vertices numbered 1,…,k-1 k Graphs I Uses only vertices numbered 1,…,k-1 CSC311: Data Structures 46 Topological Sorting Number vertices, so that (u,v) in E implies u < v wake up 1 A typical student day 3 2 study computer sci. eat 4 7 play nap 5 more c.s. 8 write c.s. program 6 work out 9 make cookies for professors 10 11 sleep dream about graphs Graphs I CSC311: Data Structures 47 Algorithm for Topological Sorting Note: This algorithm is different than the one in Goodrich-Tamassia Method TopologicalSort(G) HG // Temporary copy of G n G.numVertices() while H is not empty do Let v be a vertex with no outgoing edges Label v n nn-1 Remove v from H Running time: O(n + m). How…? Graphs I CSC311: Data Structures 48 Topological Sorting Algorithm using DFS Simulate the algorithm by using depth-first search Algorithm topologicalDFS(G) Input dag G Output topological ordering of G n G.numVertices() for all u G.vertices() setLabel(u, UNEXPLORED) for all e G.edges() setLabel(e, UNEXPLORED) for all v G.vertices() if getLabel(v) = UNEXPLORED topologicalDFS(G, v) O(n+m) time. Graphs I Algorithm topologicalDFS(G, v) Input graph G and a start vertex v of G Output labeling of the vertices of G in the connected component of v setLabel(v, VISITED) for all e G.incidentEdges(v) if getLabel(e) = UNEXPLORED w opposite(v,e) if getLabel(w) = UNEXPLORED setLabel(e, DISCOVERY) topologicalDFS(G, w) else {e is a forward or cross edge} Label v with topological number n nn-1 CSC311: Data Structures 49 Topological Sorting Example Graphs I CSC311: Data Structures 50 Topological Sorting Example 9 Graphs I CSC311: Data Structures 51 Topological Sorting Example 8 9 Graphs I CSC311: Data Structures 52 Topological Sorting Example 7 8 9 Graphs I CSC311: Data Structures 53 Topological Sorting Example 6 7 8 9 Graphs I CSC311: Data Structures 54 Topological Sorting Example 6 5 7 8 9 Graphs I CSC311: Data Structures 55 Topological Sorting Example 4 6 5 7 8 9 Graphs I CSC311: Data Structures 56 Topological Sorting Example 3 4 6 5 7 8 9 Graphs I CSC311: Data Structures 57 Topological Sorting Example 2 3 4 6 5 7 8 9 Graphs I CSC311: Data Structures 58 Topological Sorting Example 2 1 3 4 6 5 7 8 9 Graphs I CSC311: Data Structures 59