Hexasort -- The Complexity of Stacking Colors on Graphs
Abstract
Many popular puzzle and matching games have been analyzed through the lens of computational complexity. Prominent examples include Sudoku, Candy Crush, and Flood-It. A common theme among these widely played games is that their generalized decision versions are NP-hard, which is often thought of as a source of their inherent difficulty and addictive appeal to human players. In this paper, we study a popular single-player stacking game commonly known as Hexasort. The game can be modelled as placing colored stacks onto the vertices of a graph, where adjacent stacks of the same color merge and vanish according to deterministic rules. We prove that Hexasort is NP-hard, even when restricted to single-color stacks and progressively more constrained classes of graphs, culminating in strong NP-hardness on trees of either bounded height or degree. Towards fixed-parameter tractable algorithms, we identify settings in which the problem becomes polynomial-time solvable and present dynamic programming algorithms.
Keywords
Cite
@article{arxiv.2603.01244,
title = {Hexasort -- The Complexity of Stacking Colors on Graphs},
author = {Linus Klocker and Simon D. Fink},
journal= {arXiv preprint arXiv:2603.01244},
year = {2026}
}