A monotonic stack maintains its elements in strictly increasing or decreasing order by popping violating elements as new ones arrive. It solves next-greater-element, daily-temperatures, and histogram problems in O(n).
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A monotonic stack maintains its elements in strictly increasing or decreasing order by popping violating elements as new ones arrive. It solves next-greater-element, daily-temperatures, and histogram problems in O(n).
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Before pushing a new element, pop elements that break the desired order, resolving answers as you pop.
Next greater element, daily temperatures, and largest rectangle in a histogram.
Each element is pushed and popped at most once, giving linear time overall.
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When a problem asks for the next or previous greater or smaller element, or for spans bounded by such elements.
O(n), because each element enters and leaves the stack at most once.
