Tripadvisor Software Engineer interview questions

An agent found my resume on Linked in then I got the phone interview. The questions are normal data structure and algo ones. The interview gave me not that much feedback so I think I have answered the questions properly. The whole process went on seemingly smoothly, so I didn't pass, quite to my surprise.

Applying and calling 4 days after. givin code test to solve within 40 mins. 2 questions. The first one about stacks and math operations. The second question is about datastructure and algorithm, how to get the middle index in an arraylist