Online Stock Span
Signals to notice
Brute force first
For each day, scan backward counting — total. Re-scans overlapping portions. That direct path helps you understand the question, but it tends to treat every possibility as brand new instead of learning from earlier steps.
The key insight
Monotonic decreasing stack of (price, span) pairs. When today's price ≥ stack top, pop and absorb that span. The accumulated span is pushed with today's price. amortized. 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 prices in decreasing order. When a new price dominates stack entries, their spans are absorbed — they'll never be relevant again because today's price overshadows them. As long as that statement keeps holding, you can trust the steps built on top of it.
Easy way to go wrong
Not accumulating spans when popping — each popped entry's span must be added to the current day's span, because those days are now covered by today. The fix is usually to return to the meaning of each move, not just the steps themselves.