Algorithms in C, Part 5: Graph Algorithms, Third Edition

Chapter Nineteen Digraphs and DAGs

WHEN WE ATTACH significance to the order in which the two vertices are specified in each edge of a graph, we have an entirely different combinatorial object known as a digraph, or directed graph. Figure 19.1 shows a sample digraph. In a digraph, the notation s-t describes an edge that goes from s to t, but provides no information about whether or not there is an edge from t to s. There are four different ways in which two vertices might be related in a digraph: no edge; an edge s-t from s to t; an edge t-s from t to s; or two edges s-t and t-s, which indicate connections in both directions. The one-way restriction is natural in many applications, easy to enforce in our implementations, and seems innocuous; ...