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

that's what I said "constant time"

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

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

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.