The solution validity is defined by the requirements.

The solution 1 doesn’t comply with the non-functional requirement “do not change the signature”. This has nothing to do with recursion but with your interview conditions.

This being said, and from an SE point of view, both algorithms are not equivalent:

• solution 2 is a tail recursion, which can easily be optimized as a loop by the compiler,
• solution 1 uses backtracking, which will be slower for this particular problem.

The solution validity is defined by the requirements.

The solution 1 doesn’t comply with the non-functional requirement “do not change the signature”. This has nothing to do with recursion but with your interview conditions.

This being said, and from an SE point of view, both algorithms are not equivalent:

• solution 2 is a tail recursion, which can easily be optimized as a loop by the compiler. It’s moreover a true recursion, that fully solves the problem based on a smaller instance of the same problem.
• solution 1 (edited, see comments) is also tail recursive when closely checking the applicable rules. But it is in reality the solution to a different problem: it’s no longer “is it a palindrome” but “is it a palindrome starting at index ...”. It is certainly a clever adaptation of an iterative solution making it recursive with the iterator as an argument. The trick of the default parameter helps to stay close to the initial requirements. But not only does it not comply with the requirements, but in addition the function could be called with an explicit index beyond the half length or even beyond the bounds, which might produce wrong results.