I just woke up (still in bed). I've not seen this question before. For about 2 seconds I thought of iterating through and creating a map and... Wait a second, if one number is missing, I can just find the sum and subtract! Admittedly, I would have had much more trouble in a high pressure situation.

I think questions like this are fun and novel and also have absolutely no place in an interview for software developers.

Edit: the slightest of arguments for a question like this is can they find novel runtime optimizations. A work sample test is better. Grab some crap code from the codebase like a double forloop that should have been a map look up and have them make it more performant.