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Lazy in-order BST iterator

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AppleCodingLast asked Feb 2026

Write an iterator over a binary search tree that returns values in sorted order one at a time, with next() and has_next(). Do it lazily with a stack, so memory is proportional to the tree's height, not its size. Follow-up: make the iterator safe when the tree is changed or the iterator is used from several threads at once.

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