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- 1. Lecture 12: Depth-First Search Steven SkienaDepartment of Computer Science State University of New York Stony Brook, NY 11794–4400http://www.cs.sunysb.edu/∼skiena
- 2. Problem of the DayProve that in a breadth-ﬁrst search on a undirected graph G,every edge in G is either a tree edge or a cross edge, where across edge (x, y) is an edge where x is neither is an ancestoror descendent of y.
- 3. Depth-First SearchDFS has a neat recursive implementation which eliminatesthe need to explicitly use a stack.Discovery and ﬁnal times are a convenience to maintain.dfs(graph *g, int v){ edgenode *p; (* temporary pointer *) int y; (* successor vertex *) if (ﬁnished) return; (* allow for search termination *) discovered[v] = TRUE; time = time + 1; entry time[v] = time; process vertex early(v); p = g− >edges[v]; while (p ! = NULL) { y = p− >y;
- 4. if (discovered[y] == FALSE) { parent[y] = v; process edge(v,y); dfs(g,y); } else if ((!processed[y]) || (g− >directed)) process edge(v,y); if (ﬁnished) return; p = p− >next; } process vertex late(v); time = time + 1; exit time[v] = time; processed[v] = TRUE;}
- 5. The Key Idea with DFSA depth-ﬁrst search of a graph organizes the edges of thegraph in a precise way.In a DFS of an undirected graph, we assign a direction to eachedge, from the vertex which discover it: 2 1 3 1 2 6 4 3 5 4 6 5
- 6. Edge Classiﬁcation for DFSEvery edge is either: 3. A Forward Edge 1. A Tree Edge to a decendant 4. A Cross Edge 2. A Back Edge to a different node to an ancestorOn any particular DFS or BFS of a directed or undirectedgraph, each edge gets classiﬁed as one of the above.
- 7. Edge Classiﬁcation Implementationint edge classiﬁcation(int x, int y){ if (parent[y] == x) return(TREE); if (discovered[y] && !processed[y]) return(BACK); if (processed[y] && (entry time[y]¿entry time[x])) return(FORWARD); if (processed[y] && (entry time[y]¡entry time[x])) return(CROSS); printf(”Warning: unclassiﬁed edge (%d,%d)”,x,y);}
- 8. DFS: Tree Edges and Back Edges OnlyThe reason DFS is so important is that it deﬁnes a very niceordering to the edges of the graph.In a DFS of an undirected graph, every edge is either a treeedge or a back edge.Why? Suppose we have a forward edge. We would haveencountered (4, 1) when expanding 4, so this is a back edge. 1 2 3 4
- 9. No Cross Edges in DFSSuppose we have a cross-edge 1 2 5 When expanding 2, we would discover 5, so the tree would look like: 3 4 6 1 2 3 4 5 6
- 10. DFS Application: Finding CyclesBack edges are the key to ﬁnding a cycle in an undirectedgraph.Any back edge going from x to an ancestor y creates a cyclewith the path in the tree from y to x.process edge(int x, int y){ if (parent[x] ! = y) { (* found back edge! *) printf(”Cycle from %d to %d:”,y,x); ﬁnd path(y,x,parent); ﬁnished = TRUE; }}
- 11. Articulation VerticesSuppose you are a terrorist, seeking to disrupt the telephonenetwork. Which station do you blow up?An articulation vertex is a vertex of a connected graph whosedeletion disconnects the graph.Clearly connectivity is an important concern in the design ofany network.Articulation vertices can be found in O(n(m+n)) – just deleteeach vertex to do a DFS on the remaining graph to see if it isconnected.
- 12. A Faster O(n + m) DFS AlgorithmIn a DFS tree, a vertex v (other than the root) is an articulationvertex iff v is not a leaf and some subtree of v has no backedge incident until a proper ancestor of v. The root is a special case since it has no ancestors. X X is an articulation vertex since the right subtree does not have a back edge to a proper ancestor. Leaves cannot be articulation vertices
- 13. Topological SortingA directed, acyclic graph has no directed cycles. B D A E C G FA topological sort of a graph is an ordering on the vertices sothat all edges go from left to right.DAGs (and only DAGs) has at least one topological sort (hereG, A, B, C, F, E, D).
- 14. Applications of Topological SortingTopological sorting is often useful in scheduling jobs in theirproper sequence. In general, we can use it to order thingsgiven precidence constraints.Example: Dressing schedule from CLR.
- 15. Example: Identifying errors in DNA fragmentassemblyCertain fragments are constrained to be to the left or right ofother fragments, unless there are errors. A B R A C A D A B R A A B R A C A B R A C A C A D A R A C A D A D A B R A C A D A D A B R A A D A B R R A C A D D A B R ASolution – build a DAG representing all the left-rightconstraints. Any topological sort of this DAG is a consistantordering. If there are cycles, there must be errors.
- 16. Topological Sorting via DFSA directed graph is a DAG if and only if no back edges areencountered during a depth-ﬁrst search.Labeling each of the vertices in the reverse order that they aremarked processed ﬁnds a topological sort of a DAG.Why? Consider what happens to each directed edge {x, y} aswe encounter it during the exploration of vertex x:
- 17. Case Analysis • If y is currently undiscovered, then we then start a DFS of y before we can continue with x. Thus y is marked completed before x is, and x appears before y in the topological order, as it must. • If y is discovered but not completed, then {x, y} is a back edge, which is forbidden in a DAG. • If y is completed, then it will have been so labeled before x. Therefore, x appears before y in the topological order, as it must.
- 18. Topological Sorting Implementationprocess vertex late(int v){ push(&sorted,v);}process edge(int x, int y){ int class; class = edge classiﬁcation(x,y); if (class == BACK) printf(”Warning: directed cycle found, not a DAG”);}We push each vertex on a stack soon as we have evaluatedall outgoing edges. The top vertex on the stack always hasno incoming edges from any vertex on the stack, repeatedlypopping them off yields a topological ordering.
- 19. Strongly Connected ComponentsA directed graph is strongly connected iff there is a directedpath between any two vertices.The strongly connected components of a graph is a partitionof the vertices into subsets (maximal) such that each subset isstrongly connected. a b e f c d g hObserve that no vertex can be in two maximal components,so it is a partition.
- 20. There is an elegant, linear time algorithm to ﬁnd the stronglyconnected components of a directed graph using DFS whichis similar to the algorithm for biconnected components.
- 21. Backtracking and Depth-First SearchDepth-ﬁrst search uses essentially the same idea as backtrack-ing.Both involve exhaustively searching all possibilities byadvancing if it is possible, and backing up as soon as thereis no unexplored possibility for further advancement. Bothare most easily understood as recursive algorithms.

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