Did you require a certain time complexity? I came across this on Leetcode. I found it really trivial to implement the “test all substrings” algorithm, but fiendishly difficult to implement the “expand around centers” algorithm.
We didn’t require any specific time or space complexity. If the candidate whipped out a suboptimal solution quickly and correctly and time permitted we might ask them about that though and how it might be improved. The purpose of the question is not to see if the candidate has memorized the answer or sees a trick, it’s so they can demonstrate basic proficiency and systematic reasoning.