Directed graph

A directed graph, vertex D cannot be reached from vertex B.

In graph theory, a directed graph (or digraph) is a graph which has directed edges. This means one end of the edge is different from the other end. Directed edges can be thought of as arrows, or as edges which can only be travelled in one direction.

Degree

For a normal graph, the degree of a vertex [math]\displaystyle{ v }[/math] is the number of edges touching [math]\displaystyle{ v }[/math].

For a digraph, there are two more definitions. The indegree of [math]\displaystyle{ v }[/math] is the number of edges going into [math]\displaystyle{ v }[/math], and the outdegree of [math]\displaystyle{ v }[/math] is the number of edges going out of [math]\displaystyle{ v }[/math].