Hamiltonian cycle and related problems : vertex-breaking, grid graphs, and Rubik's Cubes

Abstract

In this thesis, we analyze the computational complexity of several problems related to the Hamiltonian Cycle problem. We begin by introducing a new problem, which we call Tree-Residue Vertex-Breaking (TRVB). Given a multigraph G some of whose vertices are marked "breakable," TRVB asks whether it is possible to convert G into a tree via a sequence of applications of the vertex-breaking operation: disconnecting the edges at a degree-G breakable vertex by replacing that vertex with G degree-1 vertices. We consider the special cases of TRVB with any combination of the following additional constraints: G must be planar, G must be a simple graph, the degree of every breakable vertex must belong to an allowed list G, and the degree of every unbreakable vertex must belong to an allowed list G. We fully characterize these variants of TRVB as polynomially solvable or NP-complete. The TRVB problem is useful when analyzing the complexity of what could be called single-traversal problems, where some space (i.e., a configuration graph or a grid) must be traversed in a single path or cycle subject to local constraints. When proving such a problem NP-hard, a reduction from TRVB can often be used as a simpler alternative to reducing from a hard variant of Hamiltonian Cycle. Next, we analyze several variants of the Hamiltonian Cycle problem whose complexity was left open in a 2007 paper by Arkin et al [3]. That paper is a systematic study of the complexity of the Hamiltonian Cycle problem on square, triangular, or hexagonal grid graphs, restricted to polygonal, thin, super-thin, degree-bounded, or solid grid graphs. The authors solved many combinations of these problems, proving them either polynomially solvable or NP-complete, but left three combinations open. We prove two of these unsolved combinations to be NP-complete: Hamiltonian Cycle in Square Polygonal Grid Graphs and Hamiltonian Cycle in Hexagonal Thin Grid Graphs. We also consider a new restriction, where the grid graph is both thin and polygonal, and prove that the Hamiltonian Cycle problem then becomes polynomially solvable for square, triangular, and hexagonal grid graphs. Several of these results are shown by application of the TRVB results, demonstrating the usefulness of that problem. Finally, we apply the Square Grid Graph Hamiltonian Cycle problem to close a longstanding open problem: we prove that optimally solving an n x n x n Rubik's Cube is NP-complete. This improves the previous result that optimally solving an n x n x n Rubik's Cube with missing stickers is NP-complete. We prove this result first for the simpler case of the Rubik's Square -- an n x n x 1 generalization of the Rubik's Cube -- and then proceed with a similar but more complicated proof for the Rubik's Cube case.