I applied for a research position, but they decided that I am not yet qualified and offered me a software engineer position, although my background is not in computer science.

After a short phone interview with HR, which was extremely nice, I moved to the next face-to-face interview.

In the face-to-face interview, the interviewer was again super nice, and we had a pleasant conversation. Afterward, I was given a programming question in Python.

After about a week, I received a call from HR. They stated that although I answered the question and the interviewer said I was nice, they believe I need to gain more experience.

Define a function that gets a string and outputs the result of "Reverse Polish Notation" (RPN).

I had to think about what input I would like to receive and check if it is valid input.