I've been asked this question before. I don't remember if I already knew the answer or not. But....

When does this ever come up in the real world.

> You are given an array of n - 1 integers which are in the range between 1 and n. All numbers appear exactly once, except one number, which is missing. Find this missing number.

This has happened to me in real life exactly NEVER!

I think maybe I was asked you have a list of pairs of integers except one number is missing its pair. Again, no idea where I would apply this in a real world situation.