Faster Algorithms for Orienteering and $k$-TSP

Abstract

We consider the rooted orienteering problem in Euclidean space: Given $n$ points $P$ in ${\mathbb{R}}^d$, a root point $s\in P$ and a budget $\mathcal{B}>0$, find a path that starts from $s$, has total length at most $\mathcal{B}$, and visits as many points of $P$ as possible. This problem is known to be NP-hard, hence we study $(1-\delta )$-approximation algorithms. The previous Polynomial-Time Approximation Scheme (PTAS) for this problem, due to Chen and Har-Peled (2008), runs in time $n^{O(d\sqrt{d}/\delta )}(\log n{)}^{(d/\delta {)}^{O(d)}}$, and improving on this time bound was left as an open problem. Our main contribution is a PTAS with a significantly improved time complexity of $n^{O(1/\delta )}(\log n{)}^{(d/\delta {)}^{O(d)}}$. A known technique for approximating the orienteering problem is to reduce it to solving $1/\delta$ correlated instances of rooted $k$-TSP (a $k$-TSP tour is one that visits at least $k$ points). However, the $k$-TSP tours in this reduction must achieve a certain excess guarantee (namely, their length can surpass the optimum length only in proportion to a parameter of the optimum called excess) that is stronger than the usual $(1+\delta )$-approximation. Our main technical contribution is to improve the running time of these $k$-TSP variants, particularly in its dependence on the dimension $d$. Indeed, our running time is polynomial even for a moderately large dimension, roughly up to $d=O(\log \log n)$ instead of $d=O(1)$.