[aboveandbeyond]

It's actually O(1). It's (N * (N+1))/2

[gregors]

that's what I said "constant time"

[Terr_]

Your post is missing some required punctuation which would've made that easier to see.

[randomthought]

what about (n(n+1)/2)< 1000 n^2 ? .

[thedufer]

He interpreted it as "the time complexity of computing the sum of the numbers from 1 to n", which using the formula you just gave takes O(1) time.