cse101_11_9_11

cse101_11_9_11 - c. Suppose the shortest path from s to t...

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General shortest paths 1. Dijkstra’s alg a. Given G = (V, E) {le}, s For all u in V: Dist[u] = inf Dist[s] = 0 H = V While H != 0 U = node in H with smallest dist[] Remove u from H For all (u, w) in E If dist[w] > dist[u] = l(u, w) Dist[w] = dist[u] + l(u, w) b. Basic principle Shortest path to any node goes through nodes that are closer. Not true with negative edges. 2. Bellman – Ford algorithm – general algorithm a. Dijkstra is basically doing a bunch of update operations: Procedure update ((u, w) in E) If dist[w] > dist[u] + l(u, w) Dist[w] = dist[u] + l(u, w) b. U w b.i. Update is safe – it will never make a dist[] value too small b.ii. If the shortest path to w has u as its second-last node, and dist[u] is correctly set, then update(u, w) will correctly set dist[w]
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Unformatted text preview: c. Suppose the shortest path from s to t is S -> u1 -> u2 -> u3 -> t Suppose we do a long sequence of update operations that include (s1, u1) , ut, t) In that order, then dist[t] will be correctly set. What sequence of updates is guaranteed to have this property? Ans: update every edge V 1 times. d. Procedure Bellman-Ford(G, l, s) For all u in V Dist[u] = inf Dist[s] = 0 Repeat V 1 times: For all c in E Update Procedure update (u, w) If dist[w] > dist[u] + l(u, w) Dist[w] = dist[u] + l(u, w) Time: O(V * E) slower than dijkstras S A B C D E 0 inf inf inf inf inf 0 10 inf inf 4 2 0 10 6 10 3 2 0 5 6 10 3 2 5 1 9 3 2 4 1 9 3 2 e....
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This note was uploaded on 01/09/2012 for the course CSE 101 taught by Professor Staff during the Spring '08 term at UCSD.

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cse101_11_9_11 - c. Suppose the shortest path from s to t...

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