Container With Most Water
Signals to notice
Brute force first
Check every pair of lines. That instinct is useful because it follows the prompt literally, but it usually keeps revisiting work the problem is begging you to organize.
The key insight
Two pointers at edges, move the shorter line inward. Once you hold onto the right piece of information from moment to moment, the problem feels less like trial and error and more like following a shape that was there all along.
What must stay true
Moving the taller line can only decrease area (width shrinks, height can't improve), so always move the shorter one. When you keep that truth intact, each local choice supports the larger solution instead of fighting it.
Easy way to go wrong
Moving the taller line — you should always move the shorter one to potentially find a taller line. When the code becomes mechanical before the idea is clear, small edge cases start breaking the whole story.