How should I practice Chatter Message Window?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Reddit phone screen problems in FastPrep.

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Problem · Design

Chatter Message Window

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● Hard

RedditFULLTIMEPHONE SCREEN

Problem statement

Process an ordered sequence of operations on messages with unique decimal IDs. Messages are loaded in strictly increasing numeric ID order across all batches.

["LOAD", id1, text1, id2, text2, ...] appends a non-empty ordered batch and produces no result.
["SAVE"] returns all current messages in ID order as [id1, text1, id2, text2, ...].
["GET_MESSAGES", id] returns the identified message, up to two preceding messages, and up to two following messages in the same flattened form. A missing ID returns an empty list.
["GET_MULTI", id1, id2, ...] unions the windows for all existing requested IDs, removes duplicate messages, and returns them in numeric ID order.
["EDIT", id, newText] replaces the current text and returns ["true"], or returns ["false"] for a missing ID.
["GET_HISTORY", id] returns the initially loaded text followed by every successful edit text in chronological order, or an empty list for a missing ID.

Return the result of every non-LOAD operation in operation order. Repeated read queries do not change state.

Function

processChatter(operations: String[][]) → String[][]

Examples

Example 1

operations = [["LOAD","1","one","2","two","3","three","4","four","5","five"],["GET_MESSAGES","3"],["GET_MESSAGES","1"],["GET_MULTI","2","5"],["SAVE"]]return = [["1","one","2","two","3","three","4","four","5","five"],["1","one","2","two","3","three"],["1","one","2","two","3","three","4","four","5","five"],["1","one","2","two","3","three","4","four","5","five"]]

The window centered at 3 reaches both boundaries. The windows for 2 and 5 overlap, so their union contains each message once.

Example 2

operations = [["LOAD","1.10","a","2.25","b","4","c"],["LOAD","5","d","9","e"],["GET_MESSAGES","4"],["EDIT","4","C2"],["GET_MESSAGES","4"],["GET_HISTORY","4"],["SAVE"]]return = [["1.10","a","2.25","b","4","c","5","d","9","e"],["true"],["1.10","a","2.25","b","4","C2","5","d","9","e"],["c","C2"],["1.10","a","2.25","b","4","C2","5","d","9","e"]]

Editing changes the current message used by reads and save, while history retains both the loaded and edited text.

Example 3

operations = [["LOAD","10","a","20","b","30","c"],["GET_MESSAGES","99"],["EDIT","99","missing"],["GET_HISTORY","99"],["EDIT","20","B"],["EDIT","20","B2"],["GET_HISTORY","20"],["GET_MULTI","10","10","99"]]return = [[],["false"],[],["true"],["true"],["b","B","B2"],["10","a","20","B2","30","c"]]

Missing IDs have total deterministic results. Repeating 10 in GET_MULTI does not duplicate its three-message boundary window.

Constraints

1 <= operations.length <= 10^4.
Across all loads there are at most 10^5 messages, and across all multi-queries there are at most 10^5 requested IDs.
Message IDs are canonical finite decimal strings with at most two fractional digits, fit after scaling by 100 in a signed 64-bit integer, are unique, and arrive in strictly increasing numeric order.
Message text contains printable ASCII characters and has length at most 200.
Every operation has a supported opcode and the exact arity described above.