Audible Software Development Engineer Intern interview questions

Applied though University and HR contacted to schedule two back-to-back interviews 45 min each. Questions are not very hard, mainly focused on linked list, hash, array, strings. Interviewers were nice and will give hint. Design a data structure contains int that supports add(int), remove(int), random() where random will give a random integer in the data structure. Run time of all three should be O(1).