lecture17 - BFS DFS IE170: Algorithms in Systems...

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Unformatted text preview: BFS DFS IE170: Algorithms in Systems Engineering: Lecture 17 Jeff Linderoth Department of Industrial and Systems Engineering Lehigh University March 2, 2007 Jeff Linderoth IE170:Lecture 17 BFS DFS Taking Stock Last Time Depth-First Search This Time: Uses of DFS Topological Sort Strongly Connected Components Jeff Linderoth IE170:Lecture 17 BFS DFS Depth-First Search DFS Input : Graph G = ( V,E ) No source vertex here. Works for undirected and directed graphs. We focus on directed graphs today... Output : Two timestamps for each node d ( v ) , f ( v ) , Output : π ( v ) , predecessor of v not on shortest path necessarily Jeff Linderoth IE170:Lecture 17 BFS DFS DFS (Initialize and Go) dfs ( V,E ) 1 for each u in V 2 do color ( u ) ← green 3 π ( u ) ← nil 4 time ← 5 for each u in V 6 do if color [ u ] = green 7 then dfs-visit ( u ) Jeff Linderoth IE170:Lecture 17 BFS DFS DFS (Visit Node—Recursive) dfs-visit ( u ) 1 color ( u ) ← yellow 2 d [ u ] ← time ++ 3 for each v in Adj [ u ] 4 do if color [ v ] = green 5 then π [ v ] ← u 6 dfs-visit ( v ) 7 8 color ( u ) ← red 9 f [ u ] = time ++ Jeff Linderoth IE170:Lecture 17 BFS DFS Parenthesis Theorem Let’s look at the intervals: [ d [ v ] ,f [ v ]] for each vertex v ∈ V . (Surely, d [ v ] < f [ v ] ) These tell us about the predecessor relationship in G π 1 If I finish exploring u before first exploring v , ( d [ u ] < f [ v ]) then v is not a descendant of u . (Or versa vice) 2 If [ d [ u ] ,f [ u ]] ⊂ [ d [ v ] ,f [ v ]] then u is a descendent of v in the DFS tree 3 If [ d [ v ] ,f [ v ]] ⊂ [ d [ u ] ,f [ u ]] then v is a descendent of u in the DFS tree Jeff Linderoth IE170:Lecture 17 BFS DFS Analysis of DFS Loop on lines 1-3 O ( | V | ) dfs-visit is called exactly once for each vertex v ( Why? ) Because the first thing you do is paint the node yellow The Loop on lines 3-6 in calls dfs-visit | Adj [ v ] | times for vertex v ....
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This note was uploaded on 08/06/2008 for the course IE 170 taught by Professor Ralphs during the Spring '07 term at Lehigh University .

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lecture17 - BFS DFS IE170: Algorithms in Systems...

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