How should I practice First Non-Repeating Character in a Stream?

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First Non-Repeating Character in a Stream

Bloomberg LP● MediumNEW GRADONSITE INTERVIEW

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Problem statement

You receive the lowercase characters of a stream one at a time. After each new character, report the earliest character in the stream prefix whose frequency is exactly one.

If the current prefix has no non-repeating character, report the empty string for that step.

Given stream, return one result for every prefix in arrival order.

Function

firstNonRepeatingAfterEachInput(stream: String) → String[]

Examples

Example 1

stream = "google"return = ["g","g","g","","l","l"]

After the second g, o is still unique; after the second o, no unique character remains until l arrives.

Example 2

stream = "aabc"return = ["a","","b","b"]

The second a removes the only candidate. Then b becomes and remains the earliest unique character.

Example 3

stream = "abc"return = ["a","a","a"]

The first character never repeats, so it stays at the front for every prefix.

Constraints

1 <= stream.length <= 100000.
stream contains only lowercase English letters.