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Flood-It with Jewelry - Characterizing the Game Complexity for Cograph Generalizations

Authors: Martin Darmüntzel, Christian Rosenke, and Mark Scheibner

Published in: LIPIcs, Volume 386, 51st International Symposium on Mathematical Foundations of Computer Science (MFCS 2026)


Abstract
Flood-It is a single-player game played on a precolored graph G, where the objective is to make G monochromatic using as few flooding moves as possible. In each move, a color c is selected and all vertices reachable from a fixed pivot vertex via a monochromatic path are recolored with c. In the free variant, the pivot may be chosen anew in every move. Deciding whether a graph can be made monochromatic in at most k moves is NP-complete for both variants, fixed and free. This hardness persists even under strong structural restrictions such as split graphs and trees. The Free Flood-It variant is generally considered more difficult than its fixed-pivot counterpart, as it remains hard on several graph classes where the latter becomes tractable, including co-comparability and AT-free graphs. Cographs, that is, P₄-free graphs, are among the few classes on which even Free Flood-It is solvable in polynomial time and therefore serve as our starting point. We consider the ten natural one-vertex extensions of P₄ - referred to as jewels - and study the complexity of both flooding games on the 1024 graph classes obtained by forbidding subsets of these graphs as induced subgraphs. Our main contribution is a polynomial-time algorithm for Free Flood-It on graphs that are free of the three jewels bull, gem, and P₅, covering 128 of the 1024 classes. In addition, we prove that both variants remain NP-complete on thin-spider graphs, which exclude the eight jewels banner, co-banner, chair, gem, house, kite, P₅, and C₅, thereby establishing hardness for 256 additional classes. Combined with known algorithms and hardness results, our work determines the complexity of both Flood-It variants for 896 of the 1024 considered graph classes.

Cite as

Martin Darmüntzel, Christian Rosenke, and Mark Scheibner. Flood-It with Jewelry - Characterizing the Game Complexity for Cograph Generalizations. In 51st International Symposium on Mathematical Foundations of Computer Science (MFCS 2026). Leibniz International Proceedings in Informatics (LIPIcs), Volume 386, pp. 40:1-40:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2026)


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@InProceedings{darmuntzel_et_al:LIPIcs.MFCS.2026.40,
  author =	{Darm\"{u}ntzel, Martin and Rosenke, Christian and Scheibner, Mark},
  title =	{{Flood-It with Jewelry - Characterizing the Game Complexity for Cograph Generalizations}},
  booktitle =	{51st International Symposium on Mathematical Foundations of Computer Science (MFCS 2026)},
  pages =	{40:1--40:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-442-0},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{386},
  editor =	{Kouck\'{y}, Michal and Petrișan, Daniela},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.MFCS.2026.40},
  URN =		{urn:nbn:de:0030-drops-274216},
  doi =		{10.4230/LIPIcs.MFCS.2026.40},
  annote =	{Keywords: Flood-It, Free Flood-It, cograph generalizations, polynomial time algorithms, NP-completeness}
}
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