  Classical parking functions can be defined in terms of drivers with preferred
parking spaces searching a linear parking lot for an open parking spot. We may
consider this linear parking lot as a collection of $n$ vertices (parking
spots) arranged in a directed path. We generalize this notion to allow for more
complicated "parking lots" and define parking functions on arbitrary directed
graphs. We then consider a relationship proved by Lackner and Panholzer between
parking functions on trees and "mapping digraphs" and we show that a similar
relationship holds when edge orientations are reversed.
