National Instruments Developer interview questions

I got the interview on career fair. and then on-campus interview and then got the offer 4 weeks later.Two people asked both behavior and technical questions.technical questions just some data structure and algorithm questions

Shuffling cards red black tree What is the hardest thing you have met in debugging? How to calculate the average of a binary tree? What is the most fantastic project you have participated in? I cannot answer the questions fluently, at last, I was rejected. But I gave the specific idea about my debugging experience. And the recruiter is a really nice person!