Approximation of Steiner Forest via the Bidirected Cut Relaxation

TL;DR

This paper introduces a new algorithm for the Steiner forest problem based on the bidirected cut relaxation, achieving the same approximation ratio as classical methods but with improved performance on certain instances.

Contribution

It presents a novel primal-dual algorithm using bidirected cut relaxation, extending the schema with two phases to better satisfy connectivity and improve approximation.

Findings

The new algorithm matches the classical approximation ratio of 2 - 1/k.

It outperforms classical algorithms on specific challenging instances.

The approach reveals deeper combinatorial insights into the Steiner forest problem.

Abstract

The classical algorithm of Agrawal, Klein and Ravi [SIAM J. Comput., 24 (1995), pp. 440-456], stated in the setting of the primal-dual schema by Goemans and Williamson [SIAM J. Comput., 24 (1995), pp. 296-317] uses the undirected cut relaxation for the Steiner forest problem. Its approximation ratio is $2-\frac{1}{k}$, where $k$ is the number of terminal pairs. A variant of this algorithm more recently proposed by K\"onemann et al. [SIAM J. Comput., 37 (2008), pp. 1319-1341] is based on the lifted cut relaxation. In this paper, we continue this line of work and consider the bidirected cut relaxation for the Steiner forest problem, which lends itself to a novel algorithmic idea yielding the same approximation ratio as the classical algorithm. In doing so, we introduce an extension of the primal-dual schema in which we run two different phases to satisfy connectivity requirements in both directions. This reveals more about the combinatorial structure of the problem. In particular, there are examples on which the classical algorithm fails to give a good approximation, but the new algorithm finds a near-optimal solution.