Google Research

Simple Label-Correcting Algorithms for Partially-Dynamic Approximate Shortest Paths in Directed Graphs

SOSA 2020

Abstract

Classical single source shortest paths algorithms work by maintaining distance estimates d : V → ℝ and performing so-called edge relaxations. We call an edge uv of weight w(uv) relaxed if d(v) ≤ d(u) + w(uv), and tense otherwise. To relax a tense edge uv means to set d(v) to d(u)+w(uv). It is known that starting from d(s) = 0, and d(v) = ∞ for all v ≠ s, and performing edge relaxations in arbitrary order until there are no more tense edges leads to d being equal to the distances from the source s. This overall idea can be extended to a very simple incremental algorithm for maintaining shortest paths.

We consider an operation which can be seen as a dual of a relaxation and study an approximate version of both operations. We show that by repeating the respective operation until convergence one obtains very simple incremental and decremental deterministic algorithms for (1 + ∊)-approximate shortest paths in directed graphs.

Specifically, we give an algorithm maintaining all-pairs approximate shortest paths in O(n^3 log n log (nW)/∊) total update time, where the graph's edge weights come from the interval [1,W]. This is two log-factors faster than the known folklore solution obtained by combining King's decremental transitive closure algorithm [King, FOCS'99] and the h-SSSP algorithm [Bernstein, SICOMP'16] for h = 2.

In addition, we give an algorithm for approximating single source shortest paths of hop-length at most h in O(mh log(nW)/∊) total time. The obtained algorithm is simpler and more efficient than Bernstein's h-SSSP algorithm [Bernstein, SICOMP'16].

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