Read Current and Historical Field Values with Expiration

Implement `replayDatabase(operations)` for an initially empty in-memory database. Each value belongs to a `(key, field)` pair. Process operations in order and return the results of all `get` and `get_when` operations in that order. ### Inputs and Output `operations` is a list of lists of strings. Each operation has one of these forms: - `["set", time, key, field, value]`: assign a value without an expiration time. - `["set_ttl", time, key, field, value, ttl]`: assign a value that is present from `time` through, but not including, `time + ttl`. - `["delete", time, key, field]`: remove the value currently present at that pair, if any. This operation produces no output. - `["get", time, key, field]`: return the value present at `time`. - `["get_when", time, key, field, at]`: return the value that was present at the historical time `at`. This reads that pair's history without changing the current database. Times and TTL durations are encoded as decimal strings. Return a list of strings. A read of an absent value returns `""`; stored values are nonempty. ### Rules and Constraints - This practice version uses strictly increasing, nonnegative operation times. For `get_when`, `0 <= at <= time`; zero is an ordinary historical time. - A write replaces the previous value at its operation time. A deletion ends the current value at its operation time. Expiration, replacement, or deletion does not erase the value's earlier history. - An expired value does not become present again after a later value expires or is deleted. A subsequent ordinary `set` has no expiration, even if an earlier value had a TTL. - Reads do not modify values, lifetimes, or operation times. A missing key or field has no value. - There are at most 100,000 operations. Operation times and historical times are at most 1,000,000,000; TTL durations are integers from 1 through 1,000,000,000. - Keys, fields, and values are case-sensitive ASCII strings of length 1 through 50. All operations are well formed. An empty operation list returns an empty list. ### Example 1 ```text operations = [ ["set_ttl", "2", "account", "status", "trial", "3"], ["get", "4", "account", "status"], ["get", "5", "account", "status"], ["get_when", "6", "account", "status", "4"], ["get_when", "7", "account", "status", "5"] ] Output: ["trial", "", "trial", ""] ``` The value is present on `[2, 5)`. A later historical read still sees it at time 4, but not at time 5. ### Example 2 ```text operations = [ ["set", "1", "item", "state", "old"], ["set_ttl", "3", "item", "state", "new", "2"], ["delete", "4", "item", "state"], ["set", "6", "item", "owner", "team"], ["get_when", "7", "item", "state", "2"], ["get_when", "8", "item", "state", "3"], ["get_when", "9", "item", "state", "4"], ["get", "10", "item", "state"], ["get", "11", "item", "owner"] ] Output: ["old", "new", "", "", "team"] ``` Deletion preserves both earlier versions. The old value does not return, and another field under the same key remains independent.