Graph Topological Sort Using Depth-First Search

In this video tutorial, you will learn how to do a topological sort on a directed acyclic graph (DAG), i.e. arrange vertices in a sequence according to dependency constraints shown by edges. The sort is done by using the depth-first search algorithm as the basis, and modifying it to label vertices with sequence numbers. The complete code is available at https://www.dropbox.com/s/71fe4ttpj27...
and the examples graphs illustrated in the video are at https://www.dropbox.com/s/6s8i9ji1wla...
and
https://www.dropbox.com/s/tasix4uihrs...

Error: At around 6:24, the graph example used has a cycle including X, C, D, P. The edge from X to C should go in the opposite direction, from C to X. Everything else works as it should.