Problem Brief

Timed Cache with Sidecar Cleanup

FULLTIMEONSITE INTERVIEW

Implement a key-value cache where each entry has its own TTL.

Support the following operations:

SET <key> <value> <ttlSeconds> <now>
GET <key> <now>
CLEANUP <now>

GET is the hot path, so it should not scan the whole cache to remove expired entries. A separate sidecar or background process may perform proactive cleanup.

Return the output lines produced by the operation batch. GET should print the current value or NULL if the key is missing or expired.

Function Description

Complete the function runTimedCache in the editor below.

runTimedCache has the following parameter:

String[] operations: cache operations executed in order

Returns

String[]: the output lines produced by the batch.

1Example 1

Input

operations = ["SET a 1 5 0", "GET a 3", "GET a 6"]

Output

["1", "NULL"]

Explanation

Key a expires at time 5. It is still valid at time 3 and expired by time 6.

2Example 2

Input

operations = ["SET a 1 5 0", "SET b 2 2 0", "CLEANUP 3", "GET a 3", "GET b 3"]

Output

["1", "NULL"]

Explanation

By time 3, key b has expired and may be removed during cleanup. Key a is still alive.

Constraints

Limits and guarantees your solution can rely on.

GET should stay cheap even when the cache is large.
Each key has a per-item TTL.
A cleanup helper may delete expired entries outside the hot path.