How does monotonic stack solve next greater element?
Reported in Dropbox interview loops. Pattern recognition question for linear-time array scans.
Interview scenario
Context for Dropbox candidates:
For each element, find the first greater element to its right.
Model answer
Try answering aloud first
Cover trade-offs, structure, and a concrete example before revealing the baseline response.
How to frame this at Dropbox: Connect your answer to measurable impact, clarity of thought, and trade-offs the team cares about. Below is a strong baseline response you can adapt with your own project examples.
Use a decreasing stack of indices. While current value is greater than stack top value, current is the next greater element for that top index, so pop and fill result.
Then push current index and continue. Every index is pushed and popped at most once, giving O(n) time. This pattern extends to stock span, daily temperatures, and histogram area problems.
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