The recruiter only mentioned that the interview would cover algorithms and data structures, with no mention of a language requirement at all. Then right at the start of the interview, the interviewer told me I could only use JavaScript or TypeScript. I said I was more comfortable in Python, but I was still made to use JS/TS. This was the first time I've ever been forced into a specific language like that.
The problem itself wasn't hard. Roughly: implement a function whose inputs are:
- a list of browser position strings (e.g. top/bottom of page)
- a list of candidate ads
- a per-user dictionary of ad state (tracking each ad's display count)
The goal was to assign ads to as many of a given user's browser positions as possible, return the assignment result, and update the state dictionary. Requirements:
- random assignment
- no repeats when there are enough ads
- can't exceed the per-ad limit configured in the candidate ads list
My TypeScript was rusty so I wrote pretty slowly, but I did finish the implementation, and the code quality was decent — I defined classes/types to represent the input/output structures. Mostly I got stuck on some TS syntax, like how to properly type a Set.
I had about 20 minutes left when I finished coding. After that the interviewer moved into API/system-design-style questions:
- how to turn this code into an API
- where to store the per-user ad state
- what errors the API should expose
- how to design the API schema
- REST or something else
- PUT or GET
- how to design the URL
No actual implementation needed, more like the API-design discussion you'd get in a system design interview.
I personally felt like this round went reasonably well overall, so getting rejected at this stage was actually pretty surprising to me. The interviewer was only an SWE III, but with 10+ years of experience (5 of those at eBay). I got the sense they tend to nitpick small mistakes.
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