Daily Temperatures
Signals to notice
Brute force first
For each day, scan forward to find the next warmer temperature. It is a fair place to begin because it matches the surface of the question, yet it does not capture the deeper structure that makes the problem simpler.
The key insight
Use a monotonic decreasing stack of indices; pop when current temp is warmer. Instead of recomputing the world every time, you preserve just enough context to let the next decision become obvious.
What must stay true
The stack holds indices of days still waiting for a warmer day, in decreasing temperature order. As long as that statement keeps holding, you can trust the steps built on top of it.
Easy way to go wrong
Storing temperatures instead of indices — you need indices to calculate the day difference. The fix is usually to return to the meaning of each move, not just the steps themselves.