How should I practice First Unique Character in a Stream?

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Problem · String

First Unique Character in a Stream

● Medium

AmazonFULLTIMEPHONE SCREEN

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Problem statement

Characters arrive in the order of the string stream. After each arrival, append the earliest character seen so far whose frequency in the processed prefix is exactly one.

The problem statement continues

Pro

Examples

Example 1

stream = "aabc"return = "a#bb"

The first a is unique, then none is unique, and b remains first after c arrives.

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