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# design

* `hasNext()` `next()` can be called independently
* for safety, call hasNext() before next()
* cant store all data in advance

<https://leetcode.com/problems/binary-search-tree-iterator> <https://leetcode.com/problems/flatten-nested-list-iterator> <https://leetcode.com/problems/peeking-iterator> <https://leetcode.com/problems/zigzag-iterator> <https://leetcode.com/problems/flatten-2d-vector> <https://leetcode.com/problems/design-compressed-string-iterator>