Directed graph

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 formula_1 is the number of edges touching formula_1.
For a digraph, there are two more definitions. The "indegree" of formula_1 is the number of edges going into formula_1, and the "outdegree" of formula_1 is the number of edges going out of formula_1.