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Assignment 03 - DFS cycle detection (Solution)

# Assignment 03 - DFS cycle detection (Solution) - The Hong...

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The Hong Kong University of Science & Technology COMP 271: Design and Analysis of Algorithms Fall 2007 Solution to Assignment 3 Question 1: Design an algorithm to determine whether an undirected graph G = ( E,V ) contains a cycle. The running time of your algorithm should be O ( V ) ( and hence should not depend on | E | ). Prove the correctness of your algorithm. Solution: Idea of the algorithm (4 points) We claim that G has a cycle if and only if any DFS of G produces a back edge. If there is a back edge ( u,v ), then clearly there is a cycle containing ( u,v ) in the graph. And if there is no back edge, then by the theorem that an edge in an undirected graph is either a tree edge or a back edge, there are only tree edges, implying that the graph is a forest, and hence is acyclic. We exploit DFS algorithm to judge the existence of back edges. The algo- rithm visit vertices on G in the order of depth first. When visiting a vertex u , the algorithm check, for each adjacent vertex v of u , whether the edge ( u,v ) is a back edge. The judgement is positive if and only if: (1) the edge goes to a gray vertex v and (2) it does not go to the predecessor of this vertex, namely

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