Reading constraints to guess the target complexity

The limits in a problem statement, called constraints, tell you how fast your solution must be. A typical judge, the system that runs and checks your code, manages roughly 10^8 simple steps per second. So work backwards from the input size n.

Up to about 20 allows O(2^n). Up to a few thousand allows O(n²). Up to 10^5 or 10^6 needs O(n log n) or O(n).

Up to 10^9 or more needs O(log n) or a formula. Reading constraints before you design tells you which ideas are worth trying, and which brute force (trying everything) will run out of time. It turns this stage from theory into a tool you use on every problem.