Well I like the index solution simply because it doesn't require creating multiple sub strings on the heap.

The problem with interview questions is they're mostly "guess what I'm thinking" games. So while you and I might be fully objectively right about which is the better solution the point is to show that you can work with the interviewer to either get them to see that or figure out what will make them happy even if it is stupid.

But to answer your exact question, no. Solution 1 is still valid. If challenged about the signature all you had to do was call isPalindrome(input, index) from isPalindrome(input). No one said you couldn't define a new function. You are still using recursion. But can you get the interviewer to see that?

Being right is a small consolation if you don't get the job.

Well I like the index solution simply because it doesn't require creating multiple sub strings on the heap.

The problem with interview questions is they're mostly "guess what I'm thinking" games. So while you and I might be fully objectively right about which is the better solution the point is to show that you can work with the interviewer to either get them to see that or figure out what will make them happy even if it is stupid.

But to answer your exact question, no. Solution 1 is still valid. If challenged about the signature all you had to do was call _isPalindrome(input, index) from isPalindrome(input). No one said you couldn't define a new function. You are still using recursion. But can you get the interviewer to see that?

Being right is a small consolation if you don't get the job.