(148064384) bfs


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(148064384) bfs

  1. 1. Breadth-First Searc hL L 1 0 B L 2 © 2004 G drich, Tamassia A Breadth-First Search C E D F 1
  2. 2. Breadth-First Searc (§ 12.3.3) Breadth-first search h BF on a graph ith n w (BFS) is a general nique for traversing te ch a graph ABF traversal o a graph G S f v rtices and edges of G e „ Determines whether G is „ Visits all the connected „ Computes the connected components of G „ Computes a spanning forest of G © 2004 G drich, Tamassia Breadth-First Search vertices and m edges S ta s O(n + m ) time ke s BF can be further extended to solve other S graph problems Find and report a p t with the minimum number of edges a h between two given vertices „ Find a simple cycle, if there is one „ 2
  3. 3. BF Algorithm The algorithm uses a S mechanism for setting and getting “labels” of vertices of and edges Algorithm BFS(G) Input graph G Output labeling of the edges and partition of the vertices of G for all u ∈ G.ertce() setLabel(u, UNEXPLORED) for all e ∈ G.edges() ∈ v i s setLabel(e, UNEXPLORED) for all v ∈ G.vertices() ∈ if getLabel(v) = UNEXPLORED BFS(G, v) © 2004 G drich, Tamassia Algorithm BFS(G, s) L L0 ← new empty L0.insertL ) bel(s, VISITED sequence se 0 be(s tLa i← ast(s) l while ,new empty sequence ← Lii.isEmpty() for ¬L all v ∈∈ i +1 Breadth-First Search for all e ∈ G.incidentEdges(v) if Li.elements getLabel(e) = ← opposite(v,e) () w← UNEXPLO if getLabel(w) = setLabel(e, DISCOVERY) RED setLabel(w, VISITED) w, UNEXPLO Li RED Li else setLabel(e, CROSS +1.insertLas ) e, i ← i +1 i t(w) 3
  4. 4. Example unexplored vertex visited vertex unexplored edge scovery edge di cross edge A A L L 1 0 G L 1 B B L C drich, Tamassia 0 D F A C E A E © 2004 L L 1 0 F A B C E Breadth-First Search D D F 4
  5. 5. Example (cont.) L L 0 B 1 L A 0 C E L L 1 0 2 © 2004 G L F drich, Tamassia C 2 D L E L C E B 1 A B L D L A A 0 B 1 L F 2 Breadth-First Search D F C E D F 5
  6. 6. Example (cont.) L L L 1 0 B C L L E L 1 0 B L 2 © 2004 G D L 1 0 A B C D A 2 L L A drich, Tamassia L F C E 2 E F D F Breadth-First Search 6
  7. 7. Properties Notation G s: Property B connecte all the vertices and 1 BFS(G, s) visits d edges Property of Gs 2 The discovery la eled by BFS(G, s) form a spanning tree edges L0 b of G Ts Property 3 s ach L B F r e vertex v in „ The p h of Ts s to v has i 1 to o edges Li L 2 at from Every path from s to v in „ le ii edges a © 2004 G drich, Tamassia st component of s inGs a has t Breadth-First Search A C E D F A C E D F 7
  8. 8. Analysi s Setting/getting a vertex/edge Each vertex is labeled twice label takes O(1) time O a UNEXPLORE aD s VISITED Each edge is s UNEXPLOREtwice labeled „ once aD once a DISCOVERYoor s CROSS Each vertex is inserted once into a sequen s r is called Method incidentEdges for each vertex „ once „ once „ once BFS the Li O + m) time providedce graph i O the adjacency list structure ( + represented by runs „ © 2004 G n that Σv n Recall deg(v) = 2m drich, Tamassia Breadth-First Search i s 8
  9. 9. Applications Using the template method pattern, we can specialize the BFS traversal of a G to solve graphthe 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 G number edges, or them wi the mi nimum t o report that no such path exists h © 2004 G drich, Tamassia f Breadth-First Search 9
  10. 10. DF vs. BF S S Applications Spanning forest, connected components, paths, cycles Shortest paths DF S √ √ L A C E © 2004 DF S G drich, Tamassia D F L 1 0 A B L 2 Breadth-First Search √ √ Biconnected components B BF S C E BF S D F 10
  11. 11. DF vs. BF (cont.) S edge (v,w) S Back s edge ,w) Cr swi i the, same level as os (v ancestor of v in wi a „ „ next vo n thew level in i s the tree of discovery r n edges the tree of discovery sn edges L A B C E © 2004 G DF S drich, Tamassia D F L 1 0 B L 2 Breadth-First Search A C E BF S D F 11