# Tag Info

Follow-up work by Holm, Rotenberg and Thorup [1] showed that there exists a reachability oracle for planar graphs of size $O(n)$ and query time $O(1)$. This is optimal also for trees (e.g., if the input is a star graph, then you need to know the orientation of every one of the $n-1$ edges). [1] Holm, Jacob, Eva Rotenberg, and Mikkel Thorup. "Planar ...