L19cs2110fa08 - More Graphs Lecture 19 CS2110 Fall 2008 2 3...

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Unformatted text preview: More Graphs Lecture 19 CS2110 Fall 2008 2 3 2 4 3 1 2 3 4 0 1 0 1 0 0 1 0 0 0 0 1 1 1 2 3 4 1 2 3 4 Representations of Graphs Adjacency List Adjacency Matrix 1 2 3 4 Adjacency Matrix or Adjacency List? n = number of vertices m = number of edges d(u) = outdegree of u Adjacency Matrix Uses space O(n 2 ) Can iterate over all edges in time O(n 2 ) Can answer Is there an edge from u to v? in O(1) time Better for dense graphs (lots of edges) Adjacency List Uses space O(m+n) Can iterate over all edges in time O(m+n) Can answer Is there an edge from u to v? in O(d(u)) time Better for sparse graphs (fewer edges) Shortest Paths in Graphs Finding the shortest (min-cost) path in a graph is a problem that occurs often Find the shortest route between Ithaca and West Lafayette, IN Result depends on our notion of cost Least mileage Least time Cheapest Least boring All of these costs can be represented as edge weights How do we find a shortest path? Dijkstras Algorithm dijkstra(s) { D[s] = 0; D[t] = c(s,t), t s; mark s; while (some vertices are unmarked) { v = unmarked node with smallest D; mark v; for (each w adjacent to v) { D[w] = min(D[w], D[v] + c(v,w)); } } 1 2 3 4 2.4 0.9 1.5 3.1 0.1 X 2.4 1.5 1 2 3 4 2.4 0.9 1.5 3.1 0.1 X 2.4 1.5 1 2 3 4 2.4 0.9 1.5 3.1 0.1 X 1.6 1.5 1 2 3 4 2.4 0.9 1.5 3.1 0.1 X 4.6 1.6 1.5 1 2 3 4 2.4 0.9 1.5 3.1 0.1 X 4.6 1.6 1.5 1 2 3 4 2.4 0.9 1.5 3.1 0.1 X 4.6 1.6 1.5 1 2 3 4 2.4 0.9 1.5 3.1 0.1 X 2.5 1.6 1.5 1 2 3 4 2.4 0.9 1.5 3.1 0.1 X 2.5 1.6 1.5 1 2 3 4 2.4 0.9 1.5 3.1 0.1 X 2.5 The following are invariants of the loop: For u X, D(u) = d(s,u) For u X and v X, d(s,u) d(s,v) For all u, D(u) is the length of the shortest path from s to u such that all nodes on the path (except possibly u) are in X Implementation: Use a priority queue for the nodes not yet taken priority is D(u) Proof of Correctness Shortest Paths for Unweighted Graphs A Special Case Use breadth-first search Time is O(n + m) in adj list representation, O(n 2 ) in adj matrix representation S B A C D E F Undirected Trees An undirected graph is a tree if there is exactly one simple path between any pair of vertices Facts About Trees |E| = |V| 1 connected no cycles In fact, any two of these properties imply the third, and imply that the graph is a tree Spanning Trees A spanning tree of a connected undirected graph (V,E) is a subgraph (V,E') that is a tree Spanning Trees A spanning tree of a connected undirected graph (V,E) is a subgraph (V,E') that is a tree Same set of vertices V E' E (V,E') is a tree Finding a Spanning Tree A subtractive method If there is a cycle, pick...
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This note was uploaded on 02/01/2010 for the course CS 2110 taught by Professor Francis during the Fall '07 term at Cornell University (Engineering School).

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L19cs2110fa08 - More Graphs Lecture 19 CS2110 Fall 2008 2 3...

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