How should I practice First Unique ID in a Stream?

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First Unique ID in a Stream

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DoorDashFULLTIMEONSITE INTERVIEW

Problem statement

Process a finite stream of operations while maintaining the earliest ID whose total occurrence count is exactly one.

[1, id] adds id to the stream.
[2] queries the earliest currently unique ID.
Return one string per query: the decimal ID, or the literal string null when no ID is unique.
An ID leaves the unique order on its second occurrence and never rejoins after later occurrences.

Function

firstUniqueResults(operations: int[][]) → String[]

Examples

Example 1

operations = [[1,2],[1,3],[2],[1,2],[2],[1,3],[2]]return = ["2","3","null"]

ID 2 is initially first. Its second occurrence removes it, leaving 3. After 3 repeats, no unique ID remains.

Example 2

operations = [[2],[1,-5],[2],[1,-5],[2]]return = ["null","-5","null"]

The first query is empty, then -5 is unique until its second occurrence.

Constraints

operations is non-empty.
Every operation is exactly [1, id] or [2].
IDs are signed integers.