Computing almost shortest paths

  • Authors:
  • Michael Elkin

  • Affiliations:
  • Weizmann Institute of Science, Rehovot, Israel

  • Venue:
  • Proceedings of the twentieth annual ACM symposium on Principles of distributed computing
  • Year:
  • 2001

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Abstract

We study the s-sources almost shortest paths (shortly, s-ASP) problem. Given an unweighted graph G = (V, E), and a subset S ⊈ V of s nodes, the goal is to compute almost shortest paths between all the pairs of nodes S x V. We devise an algorithm with running time O(¦E¦ n&rgr; + s · n1+&zgr;) for this problem that computes the paths Pu, w for all pairs (u, w) ∈ S x V such that the length of Pu, w is at most (1 + ∈) dG (u, w) + &bgr;(&zgr;, &rgr;, ∈), and &bgr;(&zgr;, &rgr;, ∈) is constant when &zgr;, &rgr; and ∈ are (one can choose arbitrarily small constants &zgr;, &rgr; and ∈).We also devise a distributed protocol for the s-ASP problem that computes the paths Pu, w as above, and has time and communication complexities of O(s · Diam(G) + n1+&zgr;/2) (resp., O(s · Diam(G) log3 n + n1+&zgr;/2 log n)) and O(¦E¦ n&rgr; + s · n1+&zgr; (resp., O(¦E¦nrgr; + s · n1+&zgr; + n1+&rgr;+&zgr;(&rgr;-&zgr;/2)/2)) in the synchronous (resp., asynchronous) setting.Our sequential algorithm, as well as the distributed protocol, is based on a novel algorithm for constructing (1 + ∈, &bgr;(&zgr;, &rgr;, ∈))-spanners of size O(n1+5), developed in this paper. This algorithm has running time of O(¦E¦ n&rgr;), which is significantly faster than the previously known algorithm of [20], whose running time is O(n2+&rgr;). We also develop the first distributed protocol for constructing (1 + ∈, &bgr;)-spanners. The time and communication complexities of this protocol are near-optimal.