It depends how your interviewer is ranking/scoring you. One interviewer might be impressed with your ability to talk through and solve the problem from first principles, where another might ding you for not instantly knowing the answer. The former type of interviewer would be happy to give you hints and prompts, the second is a stone wall. I try to be the first -- it helps assess how a candidate responds to coaching too! -- but there's a spectrum between "nudging" and "telling them the answer straight out".