LIPIcs.FUN.2024.11.pdf
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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.
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