A graph data structure is a special type of nonlinear data structure which consists of a finite number of vertices or nodes, and edges or arcs. A graph can be both directed and undirected. A directed graph clearly indicates the direction of the edges, while an undirected graph mentions the edges, not the direction. As a result, in an undirected graph, both directions of edge are considered as a single edge. In other words, we can say a graph is a pair of sets (V, E), where V is the set of vertices and E is the set of edges:

V = {A, B, C, D, E, F}

E = {AB, BC, CE, ED, EF, DB}

In a directed graph, an edge AB is different from an edge BA while in an undirected graph, both AB and BA are the same. Graphs are handy to solve lots of complex problems in the programming world. We are going to continue our discussion of graph data structures in Chapter 9, Putting Graphs into Action. In the following diagram, we have: