Missing number(s) Interview Question Redux

Question

The common interview problem of determining the missing value in a range from 1 to N has been done a thousand times over. Variations include 2 missing values up to K missing val

Answer 1

What about this?

1. create your own set containing all the numbers
2. remove the given set of numbers from your set (no need to sort)

What's left in your set are the missing numbers.