A recent interview question that I had to review was spelled like this:

Find missing int element into array 1..100

Of course at first read I got it wrong, you have only one integer to look
for into the array. So while the obvious idea was to apply classic *sorting*
techniques and minimize array traversal to handle complexity (time and
space), it turns out there’s a much simpler way to do it if you remember
your math lessons from younger. But is it that much simpler?