Examples of difficult Hamiltonian Cycle Problems I am working on implementing algorithms to solve Hamiltonian Cycle Problem. I need difficult problem graphs to test my implementations but my google-fu is weak and am unable to find any.
Please advise where I might find a set of difficult graphs for the Hamiltonian Cycle, or how to generate such graphs for testing.
TSPLIB contains some instances of Hamiltonian Cycle. Haythorpe [1] proposes a set of around a thousand instances which are claimed to be &quot;structurally difficult&quot; (not necessarily large).

Haythorpe, Michael. &quot;FHCP Challenge Set: The first set of structurally difficult instances of the Hamiltonian cycle problem.&quot; arXiv preprint arXiv:1902.10352 (2019).