Extremal catacondensed benzenoids with respect to the Mostar index

For a given graph G , the Mostar index M o ( G ) is the sum of absolute values of the differences between n u ( e ) and n v ( e ) over all edges e = u v of G , where n u ( e ) and n v ( e ) are, respectively, the number of vertices of G lying closer to vertex u than to vertex v and the number of vertices of G lying closer to vertex v than to vertex u . In this paper, the tree-type hexagonal systems (catacondensed hydrocarbons) with the least and the second least Mostar indices are determined. We also show some properties of tree-type hexagonal systems with the greatest Mostar index. And as a by-product, we determine the graph with the greatest Mostar index among tree-type hexagonal systems with exactly one full-hexagon. These results generalize some known results on extremal hexagonal chains.