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Backend Developer interview questionsCoding Test round

A 60 min interview plan with a time-boxed script, what each question is for, and the signals to score against. Key skills: Coding, Algorithms, Data Structures, Concurrency, Performance Optimization.

Coding Challenge

50 min
What this section is for

Purpose

Evaluate technical knowledge and problem-solving skills in real-world scenarios.

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Implement a function to find the longest substring without repeating characters in a given string. Requirements: - Input: A string S - Output: Length of the longest substring without repeating characters Examples: Input: "abcabcbb" Output: 3 Explanation: The answer is "abc", with the length of 3. Input: "bbbbb" Output: 1 Explanation: The answer is "b", with the length of 1. Follow-up Questions: - What is the time complexity of your solution? - How would you optimize your solution for large inputs?

What this question is for, and what to listen for

Purpose

Assess problem-solving, algorithm design, coding proficiency, and communication skills.

Signals to score

  • Clear understanding of the problem
  • Logical and structured approach
  • Explains thought process
  • Considers edge cases
  • Efficient, clean, well-formatted code
  • Tests and debugs the code effectively
  • Open to feedback and adjusts accordingly
  • Provided correct time & space complexity

Follow-up questions

  • Can you explain your thought process on how to approach the problem?
  • How do you plan to handle edge cases in your solution?
  • Can you walk me through your code and explain each step?
  • How would you test your solution?
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If you had more time, what would you change in your solution?

What this question is for, and what to listen for

Purpose

Assess the candidate's ability to reflect on their work, identify areas for improvement, and demonstrate critical thinking and problem-solving skills.

Signals to score

  • Recognizes limits in their original answer.
  • Points out clear areas to improve.
  • Suggests specific changes.
  • Talks about how changes could help performance.
  • Shares ideas clearly and logically.
  • Shows a desire to learn and improve.

Follow-up questions

  • What would you do differently if you had more time?
  • How could you optimize your solution further?
  • Are there any edge cases you would like to handle?
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Great job on the coding exercise!

What this section is for

Purpose

Provides positive reinforcement to the candidate, helping to build confidence and create a supportive atmosphere before moving into the final part of the interview.

Closing

5 min
What this section is for

Purpose

Thanks the candidate for their time, provides next steps, and leaves a positive final impression.

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Do you have any questions for me about the team or the company?

What this question is for, and what to listen for

Purpose

Gauge the candidate’s curiosity and interest in the role.

Signals to score

  • Thoughtful questions asked
  • Interest in company culture
  • Curiosity about growth expressed
  • Questions about role raised
  • Inquiries about team challenges
  • Engagement with responses shown
  • Desire to learn next steps
  • Focus on company values expressed

Follow-up questions

  • What interests you most about this team?
  • Are there any areas of the role you’d like to explore further?
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Interview Template
Position
Backend Developer
Round
Coding Test for 60 min
Key skills
Coding, Algorithms, Data Structures, Concurrency, Performance Optimization

Backend Developer interviews — common questions

Who is this Backend Developer interview plan for?
It is written for the interviewer, not the candidate: the hiring manager, engineer or panel member running the Coding Test round for a Backend Developer role. It gives you a 60 min script to follow in the conversation — 3 questions with what each one is for and the signals to score against — so you are not writing the round from scratch the night before.
What does the Coding Test round assess?
This round is focused on: Coding, Algorithms, Data Structures, Concurrency, Performance Optimization. It works through Coding Challenge and Closing, scoring against 22 observable signals, with follow-up prompts on all 3 questions for going deeper where an answer is thin.
How is the 60 min split up?
Coding Challenge (50 min), Closing (5 min). The timings are there so the round stays on schedule and every candidate gets the same shape of interview — which is what makes two candidates comparable afterwards.
What other rounds should I run for a Backend Developer?

A single round does not cover a whole role. The other rounds in this library for a Backend Developer: