How to answer: Given a string, return the first non-repeating character and explain the complexity.

What the interviewer is testing.

Hash maps, edge cases, complexity. Whether you reach for the right structure and handle the edge cases without being prompted.

An outline for this question.

1. Ask about the character set and what to return when every character repeats.
2. Count each character once with a hash map, then scan the string in order for the first count of one.
3. State the cost: two passes, O(n) time, and O(k) space where k is the number of distinct characters.
4. Test an empty string, a string with no unique character, and one where the answer is the last character.

The mistake to avoid.

Comparing every pair of characters. It works, but it is O(n squared) and signals you did not think about a map.

The general method: clarify, plan, code, test.

The same shape works for most coding questions, whatever the topic.

• Restate the problem and ask about inputs, size limits and edge cases before you write anything.
• Describe your approach and its complexity out loud, then check it makes sense with the interviewer.
• Write clean code in small steps, narrating as you go.
• Walk through a normal case and an edge case by hand, then say what you would test.

Practice it, out loud.

Reading an outline is not the same as saying it. Answer this question in a mock interview, in your own words, then compare it with the outline above.

Practice this questionSee live interview help

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This is a Replysis practice question. It is not a leaked or official question from any company, and the outline is a guide to structure, not a script. Use your own experience and facts.