I was contacted by a recruiter via LinkedIn, followed by a 30-minute phone screening with him. A few days later, I had a 30-minute phone call with the hiring manager. He then informed me on the call that he would like me to come for onsite interviews.

After a week, my onsite interviews were scheduled. I had four interviews: three half-hour sessions and one one-hour group discussion. Two interviews were purely technical, while the other two mixed behavioral and technical questions.

How would you achieve concurrency without using threads?