How should I practice In-Memory Database with TTL and Historical Queries?

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

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In-Memory Database with TTL and Historical Queries

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Problem statement

Implement an in-memory database that stores string values by record key and field. Process the queries in order and return one string for each query. This practice interface combines the four progressively unlocked levels reported in a Zip backend technical interview: field operations, filtered scans, expiration, and historical lookup.

Every query starts with an operation name and a strictly increasing integer timestamp written as a string. Supported operations are:

["SET", time, key, field, value]: create or replace a field; return "".
["GET", time, key, field]: return its current value, or "" if absent.
["DELETE", time, key, field]: remove a currently active field; return "true" when removed and "false" otherwise.
["SCAN", time, key]: list active fields of the record in lexicographic field order as field(value), joined by ", ". Return "" for no fields.
["SCAN_PREFIX", time, key, prefix]: use the same format, keeping only fields whose names begin with prefix.
["SET_TTL", time, key, field, value, ttl]: set a field that is active from time through times strictly less than time + ttl; return "".
["GET_WHEN", time, key, field, at]: return the value that was active at past timestamp at, or "" if none was active then. The lookup itself does not change the database.

A later SET or SET_TTL replaces the previous version immediately. Expiration and deletion leave earlier versions available to GET_WHEN. The time of a GET_WHEN query may be later than its at argument.

Function

processQueries(queries: String[][]) → String[]

Examples

Example 1

queries = [["SET","1","user","name","Ada"],["SET","2","user","city","SF"],["GET","3","user","name"],["SCAN_PREFIX","4","user","c"],["SET","5","user","name","Bea"],["SCAN","6","user"],["DELETE","7","user","city"],["GET","8","user","city"],["DELETE","9","user","city"]]return = ["","","Ada","city(SF)","","city(SF), name(Bea)","true","","false"]

The update replaces the name at time 5. The first delete removes the city; the second finds no active city.

Example 2

queries = [["SET_TTL","1","r","status","open","3"],["GET","3","r","status"],["GET","4","r","status"],["SET","5","r","status","closed"],["GET_WHEN","6","r","status","2"],["GET_WHEN","7","r","status","4"],["GET_WHEN","8","r","status","5"],["SCAN","9","r"]]return = ["","open","","","open","","closed","status(closed)"]

The first value expires exactly at time 4. Historical queries recover time 2 and correctly leave time 4 empty.

Constraints

1 <= queries.length <= 200.
Query timestamps are positive and strictly increasing. Every at is nonnegative and no greater than its query's timestamp.
Keys, fields, prefixes, and values are nonempty strings of at most 30 letters or digits; values never contain parentheses or commas.
Each ttl is a positive integer. Timestamps and TTLs are at most 10^9.
Every query has exactly the arguments shown for its operation.