Computing almost shortest paths

  • Authors:
  • Michael Elkin

  • Affiliations:
  • Yale University, New Haven, CT

  • Venue:
  • ACM Transactions on Algorithms (TALG)
  • Year:
  • 2005

Quantified Score

Hi-index 0.00

Visualization

Abstract

We study the s-sources almost shortest paths(abbreviated s-ASP) problem. Given an unweightedgraph G = (V,E),and a subset S ⊆ Vof s nodes, the goal is to compute almostshortest paths between all the pairs of nodes S× V. We devise an algorithm withrunning timeO(∣E∣nρ+ s ·n1 + ζ)for this problem that computes the pathsPu,wfor all pairs (u,w) ∈S × V such that thelength ofPu,wis at most (1 + ε)dG(u,w)+ β(ζ,ρ,ε), andβ(ζ,ρ,ε) is constant whenζ, ρ, and ε are arbitrarily smallconstants.We also devise a distributed protocol for thes-ASP problem that computes the pathsPu,was above, and has time and communication complexities ofO(s ·Diam(G) +n1 +ζ/2) (respectively,O(s ·Diam(G) log3n + n1+ ζ/2 log n)) andO(∣E∣nρ +s · n1+ ζ) (respectively,O(∣E∣nρ +s · n1+ ζ +n1 + ρ +ζ(ρ − ζ/2)/2)) in thesynchronous (respectively asynchronous) setting.Our sequential algorithm, as well as the distributed protocol,is based on a novel algorithm for constructing (1 +ε, β(ζ,ρ, ε))-spannersof size O(n1+ ζ), developed in this article. Thisalgorithm has running time ofO(∣E∣nρ), which issignificantly faster than the previously known algorithm given inElkin and Peleg [2001], whose running time isÕ(n2+ ρ). We also develop the firstdistributed protocol for constructing (1 +ε,β)-spanners. The communication complexity ofthis protocol is near optimal.