TarjanDFS - SIAM J. COMPUT. Vol. 1, No. 2, June 1972...

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SIAM J. COMPUT. Vol. 1, No. 2, June 1972 DEPTH-FIRST SEARCH AND LINEAR GRAPH ALGORITHMS* ROBERT TARJAN" Abstract. The value of depth-first search or "bacltracking" as a technique for solving problems is illustrated by two examples. An improved version of an algorithm for finding the strongly connected components of a directed graph and ar algorithm for finding the biconnected components of an un- direct graph are presented. The space and time requirements of both algorithms are bounded by k 1V + k2E d- k for some constants kl, k2, and k a, where Vis the number ofvertices and E is the number of edges of the graph being examined. Key words. Algorithm, backtracking, biconnectivity, connectivity, depth-first, graph, search, spanning tree, strong-connectivity. 1. Introduction. Consider a graph G, consisting of a set of vertices U and a set of edges g. The graph may either be directed (the edges are ordered pairs (v, w) of vertices; v is the tail and w is the head of the edge) or undirected (the edges are unordered pairs of vertices, also represented as (v, w)). Graphs form a suitable abstraction for problems in many areas; chemistry, electrical engineering, and sociology, for example. Thus it is important to have the most economical algo- rithms for answering graph-theoretical questions. In studying graph algorithms we cannot avoid at least a few definitions. These definitions are more-or-less standard in the literature. (See Harary [3], for instance.) If G (, g) is a graph, a path p’v w in G is a sequence ofvertices and edges leading from v to w. A path is simple if all its vertices are distinct. A path p’v v is called a closed path. A closed path p’v v is a cycle if all its edges are distinct and the only vertex to occur twice in p is v, which occurs exactly twice. Two cycles which are cyclic permutations of each other are considered to be the same cycle. The undirected version of a directed graph is the graph formed by converting each edge of the directed graph into an undirected edge and removing duplicate edges. An undirected graph is connected if there is a path between every pair of vertices. A (directed rooted) tree T is a directed graph whose undirected version is connected, having one vertex which is the head of no edges (called the root), and such that all vertices except the root are the head of exactly one edge. The relation "(v, w) is an edge of T" is denoted by v- w. The relation "There is a path from v to w in T" is denoted by v w. If v - w, v is the father of w and w is a son of v. If v w, v is an ancestor of w and w is a descendant of v. Every vertex is an ancestor and a descendant of itself. If v is a vertex in a tree T, T is the subtree of T having as vertices all the descendants of v in T. If G is a directed graph, a tree T is a spanning tree of G if T is a subgraph of G and T contains all the vertices of G. If R and S
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TarjanDFS - SIAM J. COMPUT. Vol. 1, No. 2, June 1972...

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