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Analysis of Algorithms CS 477/677

Analysis of Algorithms CS 477/677. Instructor: Monica Nicolescu Lecture 19. Graphs. Applications that involve not only a set of items, but also the connections between them. Maps. Schedules. Computer networks. Hypertext. Circuits. 2. 2. 1. 1. 2. 1. 3. 4. 3. 4. 3. 4.

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Analysis of Algorithms CS 477/677

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  1. Analysis of AlgorithmsCS 477/677 Instructor: Monica Nicolescu Lecture 19

  2. Graphs • Applications that involve not only a set of items, but also the connections between them Maps Schedules Computer networks Hypertext Circuits CS 477/677 - Lecture 19

  3. 2 2 1 1 2 1 3 4 3 4 3 4 Graphs - Background Graphs = a set of nodes (vertices) with edges (links) between them. Notations: • G = (V, E) - graph • V = set of vertices V = n • E = set of edges E = m Directed graph Undirected graph Acyclic graph CS 477/677 - Lecture 19

  4. 2 1 3 4 2 1 2 1 9 4 8 6 3 3 4 7 4 Other Types of Graphs • A graph is connected if there is a path between every two vertices • A bipartite graph is an undirected graph G = (V, E) in which V = V1 + V2 and there are edges only between vertices in V1 and V2 Connected Not connected CS 477/677 - Lecture 19

  5. 2 1 3 5 4 Graph Representation • Adjacency list representation of G = (V, E) • An array of V lists, one for each vertex in V • Each list Adj[u] contains all the vertices v such that there is an edge between u and v • Adj[u] contains the vertices adjacent to u (in arbitrary order) • Can be used for both directed and undirected graphs 1 2 3 4 5 Undirected graph CS 477/677 - Lecture 19

  6. 2 1 2 1 3 5 4 3 4 Properties of Adjacency List Representation • Sum of the lengths of all the adjacency lists • Directed graph: • Edge (u, v) appears only once in u’s list • Undirected graph: • u and v appear in each other’s adjacency lists: edge (u, v) appears twice Directed graph E  2 E  Undirected graph CS 477/677 - Lecture 19

  7. 2 1 2 1 3 5 4 3 4 Properties of Adjacency List Representation • Memory required • (|V| + |E|) • Preferred when • the graph is sparse: E  << V 2 • Disadvantage • no quick way to determine whether there is an edge between node u and v • Time to list all vertices adjacent to u: • (degree(u)) • Time to determine if (u, v)  E: • O(degree(u)) Undirected graph Directed graph CS 477/677 - Lecture 19

  8. 0 1 0 1 0 0 1 1 1 1 2 1 0 1 0 1 0 3 1 0 1 0 1 5 4 1 1 0 0 1 Graph Representation • Adjacency matrix representation of G = (V, E) • Assume vertices are numbered 1, 2, … V  • The representation consists of a matrix A V x V : • aij = 1 if (i, j)  E 0 otherwise 1 2 3 4 5 For undirected graphs matrix A is symmetric: aij = aji A = AT 1 2 3 4 Undirected graph 5 CS 477/677 - Lecture 19

  9. Properties of Adjacency Matrix Representation • Memory required • (|V|2), independent on the number of edges in G • Preferred when • The graph is dense: E is close to V 2 • We need to quickly determine if there is an edge between two vertices • Time to list all vertices adjacent to u: • (|V|) • Time to determine if (u, v)  E: • (1) CS 477/677 - Lecture 19

  10. Weighted Graphs • Weighted graphs = graphs for which each edge has an associated weight w(u, v) w:E R, weight function • Storing the weights of a graph • Adjacency list: • Store w(u,v) along with vertex v in u’s adjacency list • Adjacency matrix: • Store w(u, v) at location (u, v) in the matrix CS 477/677 - Lecture 19

  11. Searching in a Graph • Graph searching = systematically follow the edges of the graph so as to visit the vertices of the graph • Two basic graph searching algorithms: • Breadth-first search • Depth-first search • The difference between them is in the order in which they explore the unvisited edges of the graph • Graph algorithms are typically elaborations of the basic graph-searching algorithms CS 477/677 - Lecture 19

  12. Breadth-First Search (BFS) • Input: • A graph G = (V, E) (directed or undirected) • A source vertex s  V • Goal: • Explore the edges of G to “discover” every vertex reachable from s, taking the ones closest to s first • Output: • d[v] = distance (smallest # of edges) from s to v, for all v  V • A “breadth-first tree” rooted at s that contains all reachable vertices CS 477/677 - Lecture 19

  13. 2 2 2 1 1 1 3 3 3 5 5 5 4 4 4 Breadth-First Search (cont.) • Keeping track of progress: • Color each vertex in either white, gray or black • Initially, all vertices are white • When being discovered a vertex becomes gray • After discovering all its adjacent vertices the node becomes black • Use FIFO queue Q to maintain the set of gray vertices source CS 477/677 - Lecture 19

  14. 2 1 3 5 4 Breadth-First Tree • BFS constructs a breadth-first tree • Initially contains the root (source vertex s) • When vertex v is discovered while scanning the adjacency list of a vertex u vertex v and edge (u, v) are added to the tree • u is the predecessor (parent) of v in the breadth-first tree • A vertex is discovered only once  it has only one parent source CS 477/677 - Lecture 19

  15. 2 1 3 5 4 BFS Additional Data Structures • G = (V, E) represented using adjacency lists • color[u] – the color of the vertex for all u  V • [u] – predecessor of u • If u = s (root) or node u has not yet been discovered  [u] = NIL • d[u] – the distance from the source s to vertex u • Use a FIFO queue Q to maintain the set of gray vertices source d=1 =1 d=2 =2 d=1 =1 d=2 =5 CS 477/677 - Lecture 19

  16. r r r s s s t t t u u u   0             v v v w w w x x x y y y BFS(V, E, s) • for each u  V - {s} • docolor[u] WHITE • d[u] ←  • [u] = NIL • color[s]  GRAY • d[s] ← 0 • [s] = NIL • Q  • Q ← ENQUEUE(Q, s) Q: s CS 477/677 - Lecture 19

  17. r s t u r r s s t t u u  0    1 0 0           1 1     v w x y v v w w x x y y BFS(V, E, s) • while Q  • do u ← DEQUEUE(Q) • for each v  Adj[u] • do if color[v] = WHITE • thencolor[v] = GRAY • d[v] ← d[u] + 1 • [v] = u • ENQUEUE(Q, v) • color[u] BLACK Q: s Q: w Q: w, r CS 477/677 - Lecture 19

  18. r s t u r s t u r s t u  0   1 0   1 0 2       1    1 2  v w x y v w x y v w x y r s t u r s t u r s t u 1 0 2  1 0 2 3 1 0 2 3 2 1 2  2 1 2  2 1 2 3 v w x y v w x y v w x y r s t u r s t u r s t u 1 0 2 3 1 0 2 3 1 0 2 3 2 1 2 3 2 1 2 3 2 1 2 3 v w x y v w x y v w x y Example Q: s Q: w, r Q: r, t, x Q: t, x, v Q: x, v, u Q: v, u, y Q: u, y Q: y Q:  CS 477/677 - Lecture 19

  19. Analysis of BFS • for each u  V - {s} • do color[u]  WHITE • d[u] ←  • [u] = NIL • color[s]  GRAY • d[s] ← 0 • [s] = NIL • Q   • Q ← ENQUEUE(Q, s) O(|V|) (1) CS 477/677 - Lecture 19

  20. Scan Adj[u] for all vertices u in the graph • Each vertex u is processed only once, when the vertex is dequeued • Sum of lengths of all adjacency lists = (|E|) • Scanning operations: O(|E|) Analysis of BFS • while Q  • do u ← DEQUEUE(Q) • for each v  Adj[u] • do if color[v] = WHITE • thencolor[v] = GRAY • d[v] ← d[u] + 1 • [v] = u • ENQUEUE(Q, v) • color[u] BLACK • Total running time for BFS = O(|V| + |E|) (1) (1) CS 477/677 - Lecture 19

  21. r s t u 1 0 2 3 2 1 2 3 v w x y Shortest Paths Property • BFS finds the shortest-path distance from the source vertex s  V to each node in the graph • Shortest-path distance = (s, u) • Minimum number of edges in any path from s to u source CS 477/677 - Lecture 19

  22. 2 1 3 5 4 Depth-First Search • Input: • G = (V, E) (No source vertex given!) • Goal: • Explore the edges of G to “discover” every vertex in V starting at the most current visited node • Search may be repeated from multiple sources • Output: • 2 timestamps on each vertex: • d[v] = discovery time • f[v] = finishing time (done with examining v’s adjacency list) • Depth-first forest CS 477/677 - Lecture 19

  23. 2 1 3 5 4 Depth-First Search • Search “deeper” in the graph whenever possible • Edges are explored out of the most recently discovered vertex v that still has unexplored edges • After all edges of v have been explored, the search “backtracks” from the parent of v • The process continues until all vertices reachable from the original source have been discovered • If undiscovered vertices remain, choose one of them as a new source and repeat the search from that vertex • DFS creates a “depth-first forest” CS 477/677 - Lecture 19

  24. 1 ≤ d[u] < f [u] ≤ 2 |V| GRAY BLACK WHITE 0 d[u] f[u] 2|V| DFS Additional Data Structures • Global variable: time-step • Incremented when nodes are discovered/finished • color[u] – similar to BFS • White before discovery, gray while processing and black when finished processing • [u] – predecessor of u • d[u], f[u] – discovery and finish times CS 477/677 - Lecture 19

  25. u v w x y z DFS(V, E) • for each u  V • do color[u] ← WHITE • [u] ← NIL • time ← 0 • for each u  V • do if color[u] = WHITE • then DFS-VISIT(u) • Every time DFS-VISIT(u) is called, u becomes the root of a new tree in the depth-first forest CS 477/677 - Lecture 19

  26. u v w u u v v w w 1/ 2/ 1/ x y z x x y y z z DFS-VISIT(u) • color[u] ← GRAY • time ← time+1 • d[u] ← time • for each v  Adj[u] • do if color[v] = WHITE • then [v] ← u • DFS-VISIT(v) • color[u] ← BLACK • time ← time + 1 • f[u] ← time time = 1 CS 477/677 - Lecture 19

  27. u u v w v w u v w u u u u v v v v w w w w 1/ 2/ 1/ 2/ 1/ 1/ 1/ 1/ 1/ 2/7 2/ 2/ 2/7 3/ 4/5 4/5 4/5 4/5 3/6 3/6 3/6 3/ x y z x y z x y z x x x x y y y y z z z z u u v w v w 1/ 2/ 1/ 2/ B B B B B 4/ 3/ 4/ 3/ x y z x y z F Example CS 477/677 - Lecture 19

  28. u u u u u u u v v v v v v v w w w w w w w 1/8 1/8 1/8 1/8 1/8 1/8 1/8 2/7 2/7 2/7 2/7 2/7 2/7 2/7 9/ 9/ 9/12 9/ 9/ 9/ C C C C C 4/5 4/5 4/5 4/5 4/5 4/5 4/5 3/6 3/6 3/6 3/6 3/6 3/6 3/6 10/ 10/11 10/11 10/ x x x x x x x y y y y y y y z z z z z z z B B B B B B B B B B F F F F F F F Example (cont.) • The results of DFS may depend on: • The order in which nodes are explored in procedure DFS • The order in which the neighbors of a vertex are visited in DFS-VISIT CS 477/677 - Lecture 19

  29. u v w u v w 1/ 1/ 2/ 4/ 3/ x y z x y z B Edge Classification • Tree edge (reaches a WHITE vertex): • (u, v) is a tree edge if v was first discovered by exploring edge (u, v) • Back edge (reaches a GRAY vertex): • (u, v), connecting a vertex u to an ancestor v in a depth first tree • Self loops (in directed graphs) are also back edges CS 477/677 - Lecture 19

  30. u u v v w w 1/8 1/ 2/7 2/7 9/ C 4/5 4/5 3/6 3/6 x x y y z z B B F F Edge Classification • Forward edge (reaches a BLACK vertex & d[u] < d[v]): • Non-tree edge (u, v) that connects a vertex u to a descendant v in a depth first tree • Cross edge (reaches a BLACK vertex & d[u] > d[v]): • Can go between vertices in same depth-first tree (as long as there is no ancestor / descendant relation) or between different depth-first trees CS 477/677 - Lecture 19

  31. (|V|) (|V|) – without counting the time for DFS-VISIT Analysis of DFS(V, E) • for each u  V • do color[u] ← WHITE • [u] ← NIL • time ← 0 • for each u  V • do if color[u] = WHITE • then DFS-VISIT(u) CS 477/677 - Lecture 19

  32. Each loop takes |Adj[u]| (|E|) Analysis of DFS-VISIT(u) • color[u] ← GRAY • time ← time+1 • d[u] ← time • for each v  Adj[u] • do if color[v] = WHITE • then [v] ← u • DFS-VISIT(v) • color[u] ← BLACK • time ← time + 1 • f[u] ← time DFS-VISIT is called exactly once for each vertex Total: ΣuV|Adj[u]| + (|V|) = = (|V| + |E|) CS 477/677 - Lecture 19

  33. u v w 1/ 2/ 3/ x y z Properties of DFS • u = [v]  DFS-VISIT(v) was called during a search of u’s adjacency list • Vertex v is a descendant of vertex u in the depth first forest  v is discovered during the time in which u is gray CS 477/677 - Lecture 19

  34. Parenthesis Theorem y z s t In any DFS of a graph G, for all u, v, exactly one of the following holds: • [d[u], f[u]] and [d[v], f[v]] are disjoint, and neither of u and v is a descendant of the other • [d[v], f[v]] is entirely within [d[u], f[u]] and v is a descendant of u • [d[u], f[u]] is entirely within [d[v], f[v]] and u is a descendant of v 3/6 2/9 1/10 11/16 4/5 7/8 12/13 14/15 x w v u s t v z u w y x 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 (s (z (y (x x) y) (w w) z) s) (t (v v) (u u) t) Well-formed expression: parenthesis are properly nested CS 477/677 - Lecture 19

  35. u u 1/ 1/8 2/ 2/7 9/12 C 4/5 3/6 10/11 v v B B F Other Properties of DFS Corollary Vertex v is a proper descendant of u  d[u] < d[v] < f[v] < f[u] Theorem (White-path Theorem) In a depth-first forest of a graph G, vertex v is a descendant of u if and only if at time d[u], there is a path u  v consisting of only white vertices. CS 477/677 - Lecture 19

  36. Topological Sort Topological sort of a directed acyclic graph G = (V, E): a linear order of vertices such that if there exists an edge (u, v), then u appears before v in the ordering. • Directed acyclic graphs (DAGs) • Used to represent precedence of events or processes that have a partial order a before bb before c b before c a before c What about a and b? a before c Topological sort helps us establish a total order CS 477/677 - Lecture 19

  37. socks undershorts pants shoes watch shirt belt tie jacket Topological Sort undershorts socks Topological sort: an ordering of vertices along a horizontal line so that all directed edges go from left to right. pants shoes shirt belt watch tie jacket CS 477/677 - Lecture 19

  38. Topological Sort • TOPOLOGICAL-SORT(V, E) • Call DFS(V, E) to compute finishing times f[v] for each vertex v • When each vertex is finished, insert it onto the front of a linked list • Return the linked list of vertices undershorts 11/ 16 17/ 18 socks 15 12/ pants shoes 13/ 14 1/ 8 shirt 6/ 7 belt watch 9/ 10 tie 2/ 5 jacket 3/ 4 socks undershorts pants shoes watch shirt belt tie jacket Running time: (|V| + |E|) CS 477/677 - Lecture 19

  39. Readings • Chapter 22 CS 477/677 - Lecture 19 39

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