Explain persistent/immutable data structures and their benefits.

Persistent data structures preserve previous versions after modifications, enabling immutability. Implemented via structural sharing - new versions share unchanged parts with old versions (copy-on-write). For example, persistent trees create new path from root to modified node, sharing other nodes. Benefits: thread safety without locks, easy undo/redo, functional programming compatibility, debugging with time travel. Clojure's data structures and React's immutable state management use this concept. Space overhead is often O(log n) per operation.