The correct intepretation is: vertex in E is a pair (a,b), where a,b are vertex in D. If there exists vertices (a,b) and (c,d) in E and there is an edge from (a,b) to (c,d) in E, but b!=c in D, we have a contradiction.black.phoenix wrote:

He treated a vertex in D as a pair (a,b) where a and b are its edges. But why if there's some directed edge from (a,b) to (c,d) in D guarantees that b=c is in D???

Thanks!!!

Note this line:

I know I didn't say anything in much detail, or else I'll be spoiling it. What I said is already in the problem statement. Just think about how could we use that information, and when exactly do we output "No".There are no other edges in E.