## Problem Design a `PermanentStack` class that behaves like a standard stack but never truly destroys data. When you `pop()` an element, it is moved to an internal "archive" rather than discarded. All previously popped elements must remain retrievable. ```python class PermanentStack: def push(self, val: int) -> None: ... def pop(self) -> int: ... # removes from active, archives it def peek(self) -> int: ... # top of active stack def restore(self) -> int: ... # moves most-recently archived back to active def get_archive(self) -> list[int]: ... # all archived values, oldest first ``` **Example:** ``` push(1), push(2), push(3) pop() -> 3 (archive: [3]) pop() -> 2 (archive: [3, 2]) restore() -> 2 (active top: 2, archive: [3]) get_archive() -> [3] ``` ## Follow-ups - What if `restore()` must reinstate elements in LIFO order from the archive? - How would you implement `undo_all()` to move every archived element back in original push order? - What are the time and space complexities of each operation? - How would you persist this structure to disk so it survives a process restart?

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Problem Design a PermanentStack class that behaves like a standard stack but never truly destroys data. When you pop() an element, it is moved to an internal "archive" rather than discarded. All previously popped elements must remain retrievable. Example: Follow-ups What if restore() must reinstate elements in LIFO order from the archive? How would you implement undo_all() to move every archived element back in original push order? What are the time and space complexities of each operation? How…

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Permanent Stack - Coding Interview

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