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Media Exposition

DOI: 10.4230/LIPIcs.SoCG.2025.84

Incremental and Interactive PQ- and PC-Trees (Media Exposition)

Authors: Simon D. Fink and Dominik Peters

Published in: LIPIcs, Volume 332, 41st International Symposium on Computational Geometry (SoCG 2025)

Abstract

PQ-trees [Booth, 1975; Kellogg S. Booth and George S. Lueker, 1976] (and their more general variant PC-trees [Hsu and McConnell, 2003; Wei-Kuan and Wen-Lian, 1999]) are a well-known data structure for representing the set of linear (or for PC-trees, circular) orders respecting a given set of consecutivity constraints. Each such constraint is specified by a set of elements and requires that these elements appear consecutively in the linear (or circular) order; thus, they disallow the set to be interleaved with its complement. The main operation supported by these data structures is thus the so-called update, which takes as input a set that forms an additional constraint, and in response changes the tree in order to restrict the represented orders to those satisfying the new constraint. Interpreting a given tree is straightforward: leaves represent the underlying elements, while inner nodes either allow the order of their subtrees to be reversed (Q/C-nodes) or to be arbitrarily permuted (P-nodes). However, the way this structure and the set of represented orders change under updates is less intuitive. We present an interactive web app that allows users to specify sets of consecutivity constraints in the form of a 0/1-matrix and then calculates and visualizes the corresponding PQ- or PC-tree. The constraints can then be changed dynamically while observing how this changes the structure of the tree and the set of represented orders. Through this interactive exploration, we hope to make PQ- and PC-trees more accessible to a wider audience.

Cite as

Simon D. Fink and Dominik Peters. Incremental and Interactive PQ- and PC-Trees (Media Exposition). In 41st International Symposium on Computational Geometry (SoCG 2025). Leibniz International Proceedings in Informatics (LIPIcs), Volume 332, pp. 84:1-84:4, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@InProceedings{fink_et_al:LIPIcs.SoCG.2025.84,
  author =	{Fink, Simon D. and Peters, Dominik},
  title =	{{Incremental and Interactive PQ- and PC-Trees}},
  booktitle =	{41st International Symposium on Computational Geometry (SoCG 2025)},
  pages =	{84:1--84:4},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-370-6},
  ISSN =	{1868-8969},
  year =	{2025},
  volume =	{332},
  editor =	{Aichholzer, Oswin and Wang, Haitao},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2025.84},
  URN =		{urn:nbn:de:0030-drops-232365},
  doi =		{10.4230/LIPIcs.SoCG.2025.84},
  annote =	{Keywords: PQ trees, PC trees, planarity, consecutive ones problem, interactive exploration}
}

Document

DOI: 10.4230/LIPIcs.ITCS.2019.14

Almost Envy-Free Allocations with Connected Bundles

Authors: Vittorio Bilò, Ioannis Caragiannis, Michele Flammini, Ayumi Igarashi, Gianpiero Monaco, Dominik Peters, Cosimo Vinci, and William S. Zwicker

Published in: LIPIcs, Volume 124, 10th Innovations in Theoretical Computer Science Conference (ITCS 2019)

Abstract

We study the existence of allocations of indivisible goods that are envy-free up to one good (EF1), under the additional constraint that each bundle needs to be connected in an underlying item graph G. When the items are arranged in a path, we show that EF1 allocations are guaranteed to exist for arbitrary monotonic utility functions over bundles, provided that either there are at most four agents, or there are any number of agents but they all have identical utility functions. Our existence proofs are based on classical arguments from the divisible cake-cutting setting, and involve discrete analogues of cut-and-choose, of Stromquist's moving-knife protocol, and of the Su-Simmons argument based on Sperner's lemma. Sperner's lemma can also be used to show that on a path, an EF2 allocation exists for any number of agents. Except for the results using Sperner's lemma, all of our procedures can be implemented by efficient algorithms. Our positive results for paths imply the existence of connected EF1 or EF2 allocations whenever G is traceable, i.e., contains a Hamiltonian path. For the case of two agents, we completely characterize the class of graphs G that guarantee the existence of EF1 allocations as the class of graphs whose biconnected components are arranged in a path. This class is strictly larger than the class of traceable graphs; one can check in linear time whether a graph belongs to this class, and if so return an EF1 allocation.

Cite as

Vittorio Bilò, Ioannis Caragiannis, Michele Flammini, Ayumi Igarashi, Gianpiero Monaco, Dominik Peters, Cosimo Vinci, and William S. Zwicker. Almost Envy-Free Allocations with Connected Bundles. In 10th Innovations in Theoretical Computer Science Conference (ITCS 2019). Leibniz International Proceedings in Informatics (LIPIcs), Volume 124, pp. 14:1-14:21, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2019)

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@InProceedings{bilo_et_al:LIPIcs.ITCS.2019.14,
  author =	{Bil\`{o}, Vittorio and Caragiannis, Ioannis and Flammini, Michele and Igarashi, Ayumi and Monaco, Gianpiero and Peters, Dominik and Vinci, Cosimo and Zwicker, William S.},
  title =	{{Almost Envy-Free Allocations with Connected Bundles}},
  booktitle =	{10th Innovations in Theoretical Computer Science Conference (ITCS 2019)},
  pages =	{14:1--14:21},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-095-8},
  ISSN =	{1868-8969},
  year =	{2019},
  volume =	{124},
  editor =	{Blum, Avrim},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITCS.2019.14},
  URN =		{urn:nbn:de:0030-drops-101078},
  doi =		{10.4230/LIPIcs.ITCS.2019.14},
  annote =	{Keywords: Envy-free Division, Cake-cutting, Resource Allocation, Algorithmic Game Theory}
}

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Documents authored by Peters, Dominik

Incremental and Interactive PQ- and PC-Trees (Media Exposition)

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Almost Envy-Free Allocations with Connected Bundles

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