Moderate Queue Question 99 of 224

How do you find the first unique character in a stream as characters arrive?

DSA interview set · Speak this in 60–90 seconds · Faridabad & Delhi NCR

Key"user_id"

Hashslot 17

Valuethe record

Maintain a frequency map and a queue of candidate characters (or indices).

Why interviewers care about Queue:

This is a process

question about Queue.

Panels listen for order,

trade-offs, and what you would actually do on a DSA project - not buzzwords.

Stay structured

Name the idea, why it exists, then one short example.

Close cleanly

End with when you use it and one common pitfall.

Before you speak the answer, walk the interviewer through these steps:

1
Maintain a frequency map
and a queue of candidate characters (or indices).
2
On each arrival increment
frequency and enqueue
3
then dequeue from the
front while its frequency is no longer one.
4
Amortized O(1) per character
and O(alphabet) space.
5
A linked hash of
uniques also works.
6
Close
When you pick this over the alternative.

Here's a short line you can speak, broken into clear beats:

Say this line

“Amortized O(1) per character and O(alphabet) space.”

Break into beats

AmortizedO1percharacterand

Speaking order

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Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.

Maintain a frequency map and a queue of candidate characters (or indices). On each arrival increment frequency and enqueue