Determine run-time of an algorithm Probably a stupid question but I don't get it right now. I have an algorithm with an input n. It needs n + (n-1) + (n-2) + ... + 1 steps to finish. Is it possible to give a runtime estimation in Big-O notation?

There are $n$ terms in your series, and each term is of the order $n$ at maximum.  Therefore the operation takes on the order of $O(n^2)$ steps.
