Medium

Amazon-styleMeta-style

Longest Run of Non-Repeating Characters DSA Interview

Given a string, find the length of the longest contiguous substring in which no character repeats.

Mock Interview with AI

1. Problem Statement

You are given a string. Find the length of the longest contiguous substring that contains no repeated characters.

2. Live Coding Format

Choose Python, JavaScript, Java, or C++, then solve while explaining your reasoning to the live interviewer.

Validation uses handwritten tests only. The MVP does not compile or run code, and it does not provide AI algorithm completion.

3. Key Focus Areas

1
Problem clarification and contract
2
Approach exploration and optimality
3
Think-aloud communication and pacing
4
Implementation correctness
5
Code quality and language fluency
6
Handwritten tests and dry run
7
Debugging and self-correction
8
Complexity analysis
9
Follow-up performance

4. What Strong Candidates Should Demonstrate

✓Clarify character set, case sensitivity, and empty-string behavior before coding.
✓Explain a brute-force baseline and improve to a sliding window with a last-seen-index map.
✓Think aloud while implementing readable, correct window logic.
✓Validate with explicit expected outputs and a top-to-bottom handwritten dry run.
✓Analyze time and space complexity and handle a bounded follow-up modification.

Evaluation Guide

This is an evaluation framework, not a single model answer. Strong designs may make different choices when their assumptions and trade-offs are explicit.

Problem clarification and contract

10%

Senior signals

✓Clarifies case sensitivity, allowed character set (ASCII vs. broader Unicode), and behavior for an empty string.
✓Confirms the answer should be a length, not the substring itself, unless asked otherwise.

Staff-level signals

◆Asks about expected input size to gauge whether an O(n) approach is necessary.

Common red flags

×Codes immediately while assuming ASCII-only input without confirming.

Approach exploration and optimality

20%

Senior signals

✓Explains a brute-force approach checking all substrings for uniqueness, identifies the repeated-checking bottleneck, and improves to a sliding window with a last-seen-index map.
✓Explains how the window's left boundary jumps forward when a repeat is found, rather than shrinking one step at a time.

Staff-level signals

◆Compares a last-seen-index jump versus a plain shrinking-window-with-set approach and states why one is preferable.

Common red flags

×Starts implementation without explaining an approach even after one interviewer prompt.

Think-aloud communication and pacing

10%

Senior signals

✓Explains the plan before coding and narrates the window invariant while implementing.
✓Paces the core solution to leave time for validation and the follow-up.

Common red flags

×Writes substantial code silently and cannot explain what the code is doing.

Implementation correctness

25%

Senior signals

✓Implements a correct sliding window that tracks the last seen index of each character and advances the left boundary only forward, never backward.
✓Handles an empty string, a string with all identical characters, and a string with all unique characters.
✓Provides a convincing invariant argument for why the window always contains unique characters.

Common red flags

×Allows the left boundary to move backward or miscounts the window length.
×Final code contains a material correctness bug that the candidate does not identify.

Code quality and language fluency

15%

Senior signals

✓Produces readable, language-appropriate code with clear names for window boundaries and the last-seen map.

Common red flags

×Contains off-by-one errors in window length or syntax-level incoherence that prevents credible reasoning.

Handwritten validation, dry run, and debugging

10%

Senior signals

✓Provides explicit inputs and expected outputs before tracing the code from top to bottom.
✓Covers empty string, all-same characters, all-unique characters, and repeats near the start and end.
✓Finds and fixes an error independently, then re-validates the affected path.

Common red flags

×Claims the code works without a representative trace.

Complexity analysis and follow-up

10%

Senior signals

✓States linear time and bounded auxiliary space (proportional to the character set) and compares them with the brute-force baseline.

Staff-level signals

◆Discusses behavior for very large character sets or streaming input without losing focus.

Common red flags

×Cannot state the complexity of the implemented solution.
×Fails a reasonable follow-up despite having at least five minutes and receiving bounded clarification.

Trade-offs to articulate

Checking all substrings for uniqueness is simple but costs cubic or quadratic time depending on the uniqueness check used.
A sliding window with a set requires shrinking one step at a time on a repeat, costing amortized linear time but with more constant-factor overhead.
A sliding window with a last-seen-index map lets the left boundary jump directly past the repeat, improving constant factors while remaining linear time.

Practice follow-up questions

1.Coding follow-up: modify the solution to find the length of the longest substring that contains at most two distinct characters.
2.Verbal extension: how would the approach change if the string arrived as a stream and you needed the running answer after each character?

Want interactive feedback?

Practice this problem in the live coding workspace while the interviewer probes your clarification, algorithm, implementation, complexity, and handwritten tests.

Start Interview

Core Concepts

StringsSliding WindowHash MapsComplexity Analysis