How should I practice First Unique Character in a Stream?

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

Is First Unique Character in a Stream associated with Amazon?

FastPrep tags this as a Amazon phone screen and onsite interview coding practice problem when the problem catalog has that company and stage context.

First Unique Character in a Stream

Amazon● MediumFULLTIMEPHONE SCREENONSITE INTERVIEW

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.

If the current prefix has no unique character, append #. Return the concatenation of all per-arrival answers. Comparisons are case-sensitive.

Sibling variants: same core, different skin

These four problems are sibling variants of the same first-unique pattern. Their story wrappers and query timing differ, while the core rule keeps the earliest value whose frequency is exactly one.

Longest-Waiting Unique Task
Dynamic Longest-Waiting Unique Task Queries
First User to Log In Exactly Once
First Unique Character in a Stream (current)

Function

firstUniqueAfterEach(stream: String) → String

Examples

Example 1

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

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

Example 2

stream = "zz"return = "z#"

The only character stops being unique after its second occurrence.

Example 3

stream = "abc"return = "aaa"

The earliest a remains unique throughout all three prefixes.

Constraints

1 ≤ stream.length ≤ 100000.
stream contains only ASCII letters and digits.
The sentinel # does not occur in stream.