文摘
We present a greedy algorithm for the directed Steiner tree problem (DST), where any tree rooted at any (uncovered) terminal can be a candidate for greedy choice. It will be shown that the algorithm, running in polynomial time for any constant \(l\), outputs a directed Steiner tree of cost no larger than \(2(l-1)(\ln n+1)\) times the cost of any \(l\)-restricted Steiner tree, which is such a Steiner tree in which every terminal is at most \(l\) arcs away from the root or another terminal. We derive from this result that (1) DST for a class of graphs, including quasi-bipartite graphs, in which the length of paths induced by Steiner vertices is bounded by some constant can be approximated within a factor of \(O(\log n)\), and (2) the tree cover problem on directed graphs can also be approximated within a factor of \(O(\log n)\). Keywords Approximation algorithms Steiner tree problem Directed graphs Greedy algorithm Tree cover problem