For example, I want to compare linked list and array by calculating their big O notations; can I post it here on Software Engineering as a discussion?
First, we don't do discussions here. We do specific questions with specific answers. If you are looking for a discussion, you would need to go somewhere that is more suited to having a discussion.
But, if you have a question, it depends on your specific question. If you post your specific question here, I (and others in the community) can take a look and offer more specific guidance. But in the meantime, I can offer some general guidance.
We have a very common question on how to compute big O: What is O(...) and how do I calculate it?
If you're looking for help in computing big O notation, it's likely to be closed as a duplicate of that question. We really aren't in the business of helping every individual solve their specific problem, but building a database of questions and solutions. There are several comprehensive answers that walk through the process of computing Big O for algorithms in different ways as well as linking to external resources. It should help most people figure out how to get to the answer to their particular problem.
Additionally, you may be interested in the Computer Science Stack Exchange. Algorithm analysis and complexity tends to fall more into the domain of Computer Science than Software Engineering, but it's still something that software engineers need to generally be aware of when building software systems. However, before posting there, you should always read their Help Center to understand what they expect from questions, search for any existing questions that address your concern, and if you still aren't sure, ask on their Meta site.