Ying,
(1)--(2)
| \/ |
| /\ |
(3)--(4)
Here is a simple counter example to the wrong answer:
There are four rooms, and four courses. If all exams are happening at the same time, shown above, there will be six edges in the graph, 6>4, so it's not possible. But according to the correct answer, each node has only 3 edges, so it's OK.
Hope this helps. |