Topological sorting for Directed Acyclic Graph (DAG) is a linear ordering of vertices such that for every directed edge uv, vertex u comes before v in the ordering. Topological Sorting for a graph is not possible if the graph is not a DAG.
For example, a topological sorting of the following graph is “5 4 2 3 1 0″. There can be more than one topological sorting for a graph. For example, another topological sorting of the following graph is “4 5 2 3 1 0″. The first vertex in topological sorting is always a vertex with in-degree as 0 (a vertex with no in-coming edges).