National Instruments Software Engineer interview questions

I talked to a recruiter on a college career fair and she seemed very interested. Later that day I got a call from the recruiter and she asked me if I have time to interview the following day. The interview itself was conducted by two recruiters. One of them asked all the questions and the other just took notes. Even though the interview went very well and I solved all the technical questions, I did not get an offer.

Three rounds. First round was Aptitude + Programming Concepts. Some questions were writing the output of the given code. Time is not a constraint, they give enough time to complete the test (I think it was 90 minutes for 9 questions). Second round was coding. 3 hours, 2 questions. The point here is again time is not a constraint. If you get the logic, you can finish the round within the given time. One question was quite easy, the other was of higher standard. I passed around 19/28 test cases. 11 of us were shortlisted for Personal Interview, which was the third round. In the interview, I was first asked how I could improve the codes to better my result. The interviewer was somewhat convinced. The next question, I could not answer completely. My answer used too much memory, and they wanted an answer which didn't use extra memory. The interview was very detailed, around 1 hour. The one person who was offered Full-time position had passed 28/28 test cases in the coding round. I think there was an additional HR round for him. There were two internship positions offered. I did not get in.