Starting from a(0) = 0, each term of the sequence is a(n) = a(n-1) − n but only if a(n) > 0 and it has not previously generated. In case the condition doesn't hold, a(n) = a(n-1) + n.

Print the first 250 terms of the sequence one per line.

External links: Numberphile, OEIS A005132, Rosetta Code, Wikipedia