[R-sig-Geo] Identifying adjacent polygons within larger polygon groupings

[Sean Trende]

Hello,

I am using sf for the project below, but am open to non-sf solutions. 

Given an sf data.frame of polygons sorted into n contiguous groups (e.g., a column in the df identifies each polygon as a member of group (1, . . ., n), where within each group all polygons are neighbors), is there a simple way to identify all polygons in, say, Group 5 that are neighbors to polygons in, say, Group 8 (assuming groups 5 and 8 are neighbors)? 

E.g., if I have an sf data.frame of U.S. counties with a column that identifies the state in which each county is located, is there a way to identify all counties on the border between, say, Kansas and Nebraska? 

Thank you so much,

Sean