CS300-09_Graph_and_Digraph - Graph and Digraph Sung Yong...

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Unformatted text preview: Graph and Digraph Sung Yong Shin TC Lab. CS Dept., KAIST Contents 1. Definitions and Representations 2. A Minimum Spanning Tree Algorithm 3. A Shortest-Path Algorithm 4. Traversing Graphs and Digraphs 5. Biconnected Components of a Graph 6. Strongly Connected Components of a Digraph 2 1 NIL 4 1 3 1 3 2 NIL 4 2 NIL 4 3 NIL 1. Definitions and Representations • Representations – Adjacency Matrix – Adjacency Lists – Adjacency Multi-lists 2 1 4 3 1 2 3 4 Vertex 2. Min-Cost Spanning Tree • Greedy (1) Sort E in the non-decreasing order of weights O( e log e ) n 2 log n Why? (2) Choose n – 1 edges from the sorted list of E as long as the current selected edge does not form a cycle with the previously-chosen edges O( e + eG ( n )) Kruskal’s algorithm !!! Any Better way ? 3 1 2 1 1 4 2 3 1 1 6 2 G= ( V, E, W ) weight Why not local search ? Observation 3 1 15 20 8 2 9 F G A B C D E H 1 2 2 1 2 2 1 5 2 4 1 3 5 G = ( V, E, W ) tree vertices = {A, B, C} = V T fringe vertices = {E,F,D} = V F unseen vertices = {G, H} = V U CE = {{A,D},{B,E},{C,F}} Dijkstra / Prim Algorithm D A C B E 1 4 2 3 1 5 2 7 V T = { A } V F = { C, B, D } CE = { { A, C }, { A, B }, { A, D } } 4 3 1 D A C B E 1 4 2 3 1 5 2 7 V T = { A, C } V F = { B, D, E } CE = { { A, D }, { C, B }, { C, E } } 2 7 3 D A C B E 1 4 2 3 1 5 2 7 V T = { A, B, C } V F = { D, E } CE = { { B, D }, { B, E } } 1 D A C B E 1 4 2 3 1 5 2 7 V T = { A, B, C, D } V F = { E } CE = { { D, E } } D A C B E 1 4 2 3 1 5 2 7 V T = { A, B, C, D, E } V F = Ø CE = Ø D A C B E 1 4 2 3 1 5 2 7 SI { Select an arbitrary vertex to start the tree } v T = Ø, v F = Ø, CE = Ø ; v T = v T r { v } ; Update v F and CE ; while v F ≠ Ø do ; e : = min - weight edge in CE ; Update v T , v F , CE ; end {while} No sorting !!! Why this algorithm works ? Lemma : Let G = ( V, E, W ) be a connected weighted graph and E' ⊆ E be a subset of the edges in some minimum spanning tree T = ( V, E T ) for G . Let V' be the vertices incident with edges in E' . If { x, y } is an edge of minimum weight such that x ∈ V' and y ∉ V' , then E' r { x , y } ⊆ E T ....
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CS300-09_Graph_and_Digraph - Graph and Digraph Sung Yong...

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