POJ 2762 - Going from u to v or from v to u?
Problem : Define a graph if there exists a pair (u, v) such that no way from u to v as well as no way from v to u.
Solution : Merge the graph based on strongly connected graph to get DAG G'. Search the G' in topological order. If at some time, two vertices can be search at the same time, the graph has such pair. Otherwise no.
Proof :
	1. The vertices in the same strongly connected graph must connect to each other.
	2. Any pair of vertices can go either direction from one to another if and only if there is a path covers all the vertices in G'.
	3. If there is a path covers all the vertices in G', the topological order in G' is unique.
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