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Reorder List
This one has lots of possibilities, and it's really testing whether you can explore them systematically. Interviewers love it because it reveals your structured exploration and your edge-case discipline. The gotcha is duplicates and dead ends—good pruning and ordering rules make this manageable.
Learn this pattern
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).
Meet your coaches
Talk through the problem while you code. Signed-in reps become prep memory for the next session.
Chat with Fin or Coco right now on mobile.
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Monotonic Stacks gives you a useful next rep while you are still on your phone.