Journal or Book Title
Theoretical Computer Science
Searching in partially ordered structures has been considered in the context of information retrieval and efficient tree-like indices, as well as in hierarchy based knowledge representation. In this paper we focus on tree-like partial orders and consider the problem of identifying an initially unknown vertex in a tree by asking edge queries: an edge query e returns the component of T - e containing the vertex sought for, while incurring some known cost c(e). The Tree Search Problem with Non-Uniform Cost is the following: given a tree T on n vertices, each edge having an associated cost, construct a strategy that minimizes the total cost of the identification in the worst case.
Finding the strategy guaranteeing the minimum possible cost is an NP-complete problem already for input trees of degree 3 or diameter 6. The best known approximation guarantee was an O (log n/log log log n)-approximation algorithm of Cicalese et al. (2012) .
We improve upon the above results both from the algorithmic and the computational complexity point of view: We provide a novel algorithm that provides an O (log n/log log n)-approximation of the cost of the optimal strategy. In addition, we show that finding an optimal strategy is NP-hard even when the input tree is a spider of diameter 6, i.e., at most one vertex has degree larger than 2.
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Cicalese, Ferdinando; Keszegh, Balázs; Lidicky, Bernard; Pálvölgyid, Dömötör; and Valla, Tomáš, "On the tree search problem with non-uniform costs" (2016). Mathematics Publications. 98.