Please explain me the approach to solve this question?

Note: It is not a question of any ongoing contest, It is just past interview experience question. So please help me out solving this question

Given a string of a,b, and/or? You can replace it? with a or b. task is to replace all? such that no three consecutive a or b occurs in the string. it is given that such string will always exist we needed to return the modified string.
example
2.a) input->b?aa?
output->bbaab

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just place either of the two char and check if it is valid or not

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Hey Do you know any optimal way of solving this question because if the length of string is very large like order of 10^5 then time complexity increases