We show that several pen-and-paper puzzles are NP-complete by giving polynomial-time reductions from the Hamiltonian path and Hamiltonian cycle problems on grid graphs with maximum degree 3. The puzzles include Dotchi Loop, Chains, Linesweeper, Arukone{}₃ (also called Numberlink₃), and Araf. In addition, we show that this type of proof can still be used to prove the NP-completeness of Dotchi Loop even when the available puzzle instances are heavily restricted. Together, these results suggest that this approach holds promise in general for finding NP-completeness proofs of many pen-and-paper puzzles.
@InProceedings{deurloo_et_al:LIPIcs.FUN.2024.11, author = {Deurloo, Marnix and Donkers, Mitchell and Maarse, Mieke and Rin, Benjamin G. and Schutte, Karen}, title = {{Hamiltonian Paths and Cycles in NP-Complete Puzzles}}, booktitle = {12th International Conference on Fun with Algorithms (FUN 2024)}, pages = {11:1--11:25}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-314-0}, ISSN = {1868-8969}, year = {2024}, volume = {291}, editor = {Broder, Andrei Z. and Tamir, Tami}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FUN.2024.11}, URN = {urn:nbn:de:0030-drops-199199}, doi = {10.4230/LIPIcs.FUN.2024.11}, annote = {Keywords: Hamiltonicity, NP-completeness, complexity theory, pen-and-paper puzzles} }
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