Reading constraints to pick an approach

Every good problem statement ends with a constraints section, and most beginners skip it. That's a mistake: the constraints quietly tell you how fast your solution needs to be — and therefore which approaches are even worth trying.

The rule of thumb: about 10⁸ simple steps per second

A typical judge (and a typical interviewer's expectation) allows roughly one second, and a computer does somewhere around 100 million simple operations in that time. So plug the largest n into your algorithm's complexity and check whether the result stays under about 10⁸.

n = 100,000 (10⁵)

O(n²)       → 10¹⁰ steps     ✗  about 100 seconds: too slow
O(n log n)  → 1.7 × 10⁶      ✓
O(n)        → 10⁵            ✓