in 1 test case we can't determine whether subject 3 should be earlier than 4 or 4 than 3, because there's isn't any edge that show which one should be sdutied ealrier, why answer is YES???? I think if we can't verify it answer should be NO

Verify correctness = see if there's such a pair in the first list, that goes in reverse order on the second list. For test 2 it's pairs (3 5), (5 2), (4 2), for test 1 there's no such pairs.