Find the next greater element for each item in an array using a monotonic stack.
The brute force runs O(n squared) nested scans. A monotonic stack solves it in one O(n) pass by holding only the candidates that can still become a future answer. Here is the pattern and why each element is pushed and popped exactly once.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
The brute force runs O(n squared) nested scans. A monotonic stack solves it in one O(n) pass by holding only the candidates that can still become a future answer. Here is the pattern and why each element is pushed and popped exactly once.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
