The Time Complexity of the Token Swapping Problem and Its Parallel Variants

The problems of Permutation Routing via Matching and Token Swapping are reconfiguration problems on graphs. This paper is concerned with the complexity of those problems and a colored variant. For a given graph where each vertex has a unique token on it, those problems require to find a shortest way to modify a token placement into another by swapping tokens on adjacent vertices. While all pairs of tokens on a matching can be exchanged at once in Permutation Routing via Matching, Token Swapping allows only one pair of tokens can be swapped. In the colored version, vertices and tokens are colored and the goal is to relocate tokens so that each vertex has a token of the same color. We investigate the time complexity of several restricted cases of those problems and show when those problems become tractable and remain intractable.