convert-mutate-join-string-idiom

Status: IN

String manipulation problems (`reverse-only-letters`, `reverse-string-ii`) use the convert-mutate-join pattern: `list(s)` for mutability, in-place modification, then `"".join()` — the standard Python workaround for string immutability.

Source: entries/2026/06/06/reverse-string-ii-solution.md

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