New Algorithms and Hardness for Incremental Single-Source Shortest Paths in Directed Graphs
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In the dynamic Single-Source Shortest Paths (SSSP) problem, we are given a graph G=(V,E) subject to edge insertions and deletions and a source vertex s∈ V, and the goal is to maintain the distance d(s,t) for all t∈ V. Fine-grained complexity has provided strong lower bounds for exact partially dynamic SSSP and approximate fully dynamic SSSP [ESA'04, FOCS'14, STOC'15]. Thus much focus has been directed towards finding efficient partially dynamic (1+ϵ)-approximate SSSP algorithms [STOC'14, ICALP'15, SODA'14, FOCS'14, STOC'16, SODA'17, ICALP'17, ICALP'19, STOC'19, SODA'20, SODA'20]. Despite this rich literature, for directed graphs there are no known deterministic algorithms for (1+ϵ)-approximate dynamic SSSP that perform better than the classic ES-tree [JACM'81]. We present the first such algorithm. We present a deterministic data structure for incremental SSSP in weighted digraphs with total update time Õ(n^2 log W) which is near-optimal for very dense graphs; here W is the ratio of the largest weight in the graph to the smallest. Our algorithm also improves over the best known partially dynamic randomized algorithm for directed SSSP by Henzinger et al. [STOC'14, ICALP'15] if m=ω(n^1.1). We also provide improved conditional lower bounds. Henzinger et al. [STOC'15] showed that under the OMv Hypothesis, the partially dynamic exact s-t Shortest Path problem in undirected graphs requires amortized update or query time m^1/2-o(1), given polynomial preprocessing time. Under a hypothesis about finding Cliques, we improve the update and query lower bound for algorithms with polynomial preprocessing time to m^0.626-o(1). Further, under the k-Cycle hypothesis, we show that any partially dynamic SSSP algorithm with O(m^2-ϵ) preprocessing time requires amortized update or query time m^1-o(1).
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