In a few problems, the real important “Aha” is getting the full list of corner cases. This list define the boundary of the problem, and would point out the direction, often immediately.
Of course coding test won’t give you the time. Part of the skill tested is “clarifying requirement”. This is the real meaning.
Sugg: probe the interviewer
Sugg: Gayle pointed out some techniques to create some test data to reveal the boundary