Polynomial-time algorithm for sliding tokens on trees. Demaine, E. D., Demaine, M. L., Fox-Epstein, E., Hoang, D. A., Ito, T., Ono, H., Otachi, Y., Uehara, R., & Yamada, T. In Ahn, H. & Shin, C., editors, Algorithms and Computation - ISAAC 2014, volume 8889, of Lecture Notes in Computer Science, pages 389--400. Springer International Publishing, 2014.
Polynomial-time algorithm for sliding tokens on trees [link]Paper  Polynomial-time algorithm for sliding tokens on trees [pdf]Paper  doi  abstract   bibtex   

Suppose that we are given two independent sets $I_b$ and $I_r$ of a graph such that $| I_b | = | I_r |$, and imagine that a token is placed on each vertex in $I_b$. Then, the SLIDING TOKEN problem is to determine whether there exists a sequence of independent sets which transforms $I_b$ into $I_r$ so that each independent set in the sequence results from the previous one by sliding exactly one token along an edge in the graph. This problem is known to be PSPACE-complete even for planar graphs, and also for bounded treewidth graphs. In this paper, we show that the problem is solvable for trees in quadratic time. Our proof is constructive: for a yes-instance, we can find an actual sequence of independent sets between $I_b$ and $I_r$ whose length (i.e., the number of token-slides) is quadratic. We note that there exists an infinite family of instances on paths for which any sequence requires quadratic length.


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