Amazon Software Engineer Intern interview questions

It consisted of 2 45 minutes back to back interviews. Both are skill questions. The first interview consisted of a group of jobs in which some of them had dependencies, meaning that some had to be completed first before one could start. Think of this as a graph with pointing arrows. Create an algorithm that would spit out the most efficient/best job sequence. The result would be something along these lines: first job3, job4; second job1; third job5, job6, etc. What data structure would you use and how would you represent it? Explain it with pseudocode, then write it on code. The second interview consisted of an expression tree. What structure would you use to represent this? Write code that would compute the result. Can you write it without using recursion? What's memory overhead?