Implement Undo/Redo for Block Edits

Problem
Implement undo/redo for a sequence of edit operations on a document (e.g. typing, deleting, or formatting a block). undo() reverts the most recent operation; redo() re-applies the most recently undone operation. A new edit after some undos discards the redo history.

Input / Output

• Input: a stream of edit operations, interleaved with undo() / redo() calls.
• Output: the document state (or the operation applied/reverted) after each call.

Constraints

• History is bounded — cap the number of retained operations and drop the oldest when full.
• undo() on empty history and redo() with nothing to redo are no-ops.
• Each edit must carry enough information to be reversed.

Example

type "a"; type "b"  -> "ab"
undo()              -> "a"
undo()              -> ""
redo()              -> "a"
type "c"            -> "ac"   // redo of "b" is now discarded