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Valid Parentheses
This is a stack/invariant problem: once your stack represents the right thing, the rest is routine. It's a great warm-up that builds stack invariants without a ton of moving parts. Keep the stack invariant explicit; if it breaks, everything downstream breaks too.
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Monotonic Stacks
A monotonic stack maintains elements in sorted order, popping when a new element breaks the invariant. It solves "next greater element," "daily temperatures," and histogram problems in O(n) instead of O(n²).
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Talk through the problem while you code. Signed-in reps become prep memory for the next session.
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Monotonic Stacks gives you a useful next rep while you are still on your phone.