Improved dynamic reachability algorithms for directed graphs

L. Roditty, Uri Zwick

Research output: Contribution to journalArticlepeer-review

Abstract

We obtain several new dynamic algorithms for maintaining the transitive closure of a directed graph and several other algorithms for answering reachability queries without explicitly maintaining a transitive closure matrix. Among our algorithms are: (i) A decremental algorithm for maintaining the transitive closure of a directed graph, through an arbitrary sequence of edge deletions, in $O(mn)$ total expected time, essentially the time needed for computing the transitive closure of the initial graph. Such a result was previously known only for acyclic graphs. (ii) Two fully dynamic algorithms for answering reachability queries. The first is deterministic and has an amortized insert/delete time of $O(m\sqrt{n})$, and worst-case query time of $O(\sqrt{n})$. The second is randomized and has an amortized insert/delete time of $O(m^{0.58}n)$ and worst-case query time of $O(m^{0.43})$. This significantly improves the query times of algorithms with similar update times. (iii) A fully dynamic algorithm for maintaining the transitive closure of an acyclic graph. The algorithm is deterministic and has a worst-case insert time of $O(m)$, constant amortized delete time of $O(1)$, and a worst-case query time of $O(n/\log n)$. Our algorithms are obtained by combining several new ideas, one of which is a simple sampling idea used for detecting decompositions of strongly connected components, with techniques of Even and Shiloach [J. ACM, 28 (1981), pp. 1–4], Italiano [Inform. Process. Lett., 28 (1988), pp. 5–11], Henzinger and King [Proceedings of the $36$th Annual Symposium on Foundations of Computer Science, Milwaukee, WI, 1995, pp. 664–672], and Frigioni et al. [ACM J. Exp. Algorithmics, 6 (2001), (electronic)].
Original languageAmerican English
Pages (from-to)1455-1471
JournalSIAM Journal on Computing
Volume37
Issue number5
StatePublished - 2008

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