How do you find the first non-repeating character in a string?
PICTURE THIS: ARRAY IN MEMORY
Index starts at 0. Scan once for max — O(n).
Simple meaning
Count frequencies in a hash map or array, then scan the string again and return the first character with count one.
WHY — Strings instead of guessing?
Why interviewers care about Strings:
question about Strings.
trade-offs, and what you would actually do on a DSA project - not buzzwords.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1Count frequencies in a
hash map or array, then scan the string again and return the first character with count one.
- 2Two O(n) passes and
O(1) space for a bounded alphabet, or O(k) space for k distinct characters.
- 3A single-pass linked-hash idea
exists, but two passes is what interviewers expect first.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
Count frequencies in a hash map or array, then scan the string again and return the first character with count one. Two O(n) passes and O(1) space for a bounded alphabet, or O(k) space for k distinct characters.