A job interview problem

I was requested to solve the following problem during a job interview:

You’re given a number sequence, from 1 up to N. But, this sequence is missing a number. Also, the sequence is not sorted (it’s in a random order). You have to find out which number is missing, but the limitation is that you can’t iterate over the list more than once.

The sad thing is that I wasn’t able to solve it :-(