How should I practice Windowed Key-Value Map?

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Problem · Hash Table

Windowed Key-Value Map

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Problem statement

Implement a key-value map whose entries remain active for a fixed time window. You are given the window length windowSeconds and a chronological array of commands operations. Each command begins with a nondecreasing integer timestamp in seconds.

The supported commands are:

put timestamp key value: insert or replace key with the integer value. The new value is active during [timestamp, timestamp + windowSeconds).
get timestamp key: append the active value for key to the output, or append NOT_FOUND when the key is absent or expired.
delete timestamp key: remove the key if it is active. This command produces no output.
average timestamp: append the average of all active values. Append EMPTY when no value is active. Otherwise return the exact reduced fraction numerator/denominator; when the denominator is 1, return only the numerator.

Before applying any command at time timestamp, all entries whose expiry time is at most timestamp are inactive. Replacing a key removes its previous value and expiry before adding the new entry. Return the outputs from get and average commands in order.

The interview follow-up asks how to keep frequent average queries efficient and how the structure changes when several threads call the API concurrently.

Function

processWindowedMap(operations: String[], windowSeconds: int) → String[]

Examples

Example 1

operations = ["put 0 a 10","put 1 b 20","average 2","get 5 a","average 5"]windowSeconds = 5return = ["15","NOT_FOUND","20"]

At time 2, both values are active, so the average is 30/2 = 15. Entry a expires at time 5, while b remains active until time 6.

Example 2

operations = ["put 3 x 4","put 4 x 9","get 7 x","average 7","delete 8 x","average 8"]windowSeconds = 5return = ["9","9","EMPTY"]

The second put replaces the old value and resets x's expiry to time 9. Deleting it at time 8 leaves the map empty.

Example 3

operations = ["put 0 a -2","put 0 b 5","average 0"]windowSeconds = 10return = ["3/2"]

The active sum is 3 across two entries, so the exact average is the reduced fraction 3/2.

Constraints

1 <= operations.length <= 100000.
1 <= windowSeconds <= 10^9.
Command timestamps are integers in [0, 10^9] and are nondecreasing.
Every key contains only ASCII letters, digits, underscores, and hyphens and has length from 1 through 30.
Every inserted value is an integer in [-10^9, 10^9].
The exact active-value sum fits in a signed 64-bit integer.
Every command has exactly the fields shown in the statement.