For a set of red and blue points in the plane, a minimum bichromatic spanning tree (MinBST) is a shortest spanning tree of the points such that every edge has a red and a blue endpoint. A MinBST can be computed in O(n log n) time where n is the number of points. In contrast to the standard Euclidean MST, which is always plane (noncrossing), a MinBST may have edges that cross each other. However, we prove that a MinBST is quasi-plane, that is, it does not contain three pairwise crossing edges, and we determine the maximum number of crossings. Moreover, we study the problem of finding a minimum plane bichromatic spanning tree (MinPBST) which is a shortest bichromatic spanning tree with pairwise noncrossing edges. This problem is known to be NP-hard. The previous best approximation algorithm, due to Borgelt et al. (2009), has a ratio of O(√n). It is also known that the optimum solution can be computed in polynomial time in some special cases, for instance, when the points are in convex position, collinear, semi-collinear, or when one color class has constant size. We present an O(log n)-factor approximation algorithm for the general case.
@InProceedings{a.akitaya_et_al:LIPIcs.ISAAC.2024.4, author = {A. Akitaya, Hugo and Biniaz, Ahmad and Demaine, Erik D. and Kleist, Linda and Stock, Frederick and T\'{o}th, Csaba D.}, title = {{Minimum Plane Bichromatic Spanning Trees}}, booktitle = {35th International Symposium on Algorithms and Computation (ISAAC 2024)}, pages = {4:1--4:14}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-354-6}, ISSN = {1868-8969}, year = {2024}, volume = {322}, editor = {Mestre, Juli\'{a}n and Wirth, Anthony}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ISAAC.2024.4}, URN = {urn:nbn:de:0030-drops-221319}, doi = {10.4230/LIPIcs.ISAAC.2024.4}, annote = {Keywords: Bichromatic Spanning Tree, Minimum Spanning Tree, Plane Tree} }
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