What complexity do you need? Set n, see what passes.

Judges accept roughly 10⁸ operations per second (about 100 million). Drag n to the problem's constraint and watch which complexity classes finish in time — and which fall off the cliff.

Budget assumes ~10⁸ simple operations per second and a ~1-second limit — the common competitive-programming rule of thumb. Real limits vary with the constant factor and language, so treat the borderline band (10⁷–10⁸) as "depends." This is why the same problem's constraint tells you the intended complexity: n ≤ 10⁶ points at O(n) or O(n log n), while n ≤ 20 invites O(2ⁿ).