Easy

Amazon-styleMicrosoft-style

Validate Nested Bracket Sequences DSA Interview

Given a string containing round, square, and curly brackets, determine whether every bracket is properly opened and closed in the correct nested order.

Mock Interview with AI

1. Problem Statement

You are given a string that may contain '()', '[]', and '{}' characters along with other text. Determine whether all brackets are properly matched and correctly nested.

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 which characters count as brackets and how non-bracket characters are handled before coding.
✓Explain a naive repeated-removal baseline and improve to a single-pass stack solution.
✓Think aloud while implementing readable, correct matching logic.
✓Validate with explicit expected outputs and a top-to-bottom handwritten dry run.
✓Analyze time and space complexity and pace the easy core problem to leave room for a coding follow-up.

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 whether non-bracket characters should be ignored or make the string invalid, and confirms the empty string is balanced.
✓Confirms all three bracket types must nest correctly relative to each other, not just be individually balanced.

Staff-level signals

◆Asks whether this validates user input in a production parser and what should happen on invalid input there.

Common red flags

×Starts coding without confirming how non-bracket characters are treated.

Approach exploration and optimality

20%

Senior signals

✓Explains a baseline that repeatedly scans and removes adjacent matched pairs until no more changes occur, identifies its repeated-pass bottleneck, and improves to a single-pass stack solution.
✓Explains why a stack captures the most-recent-unmatched-opening invariant needed for correct nesting.

Staff-level signals

◆Discusses how the approach generalizes to additional bracket-like pair types without hardcoding three cases.

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 stack push/pop invariant while implementing.
✓Completes the easy core problem efficiently enough to leave meaningful time for a follow-up.

Common red flags

×Writes substantial code silently and cannot explain what the code is doing.
×Spends most of the interview on the easy core problem and leaves insufficient time for a follow-up.

Implementation correctness

25%

Senior signals

✓Implements a correct stack-based single pass: pushes openings, and on a closing bracket checks the stack top matches before popping, failing fast on mismatch.
✓Handles an empty string, a string with only non-bracket characters, unmatched openings, and unmatched closings.
✓Correctly requires the stack to be empty at the end for a balanced result.

Common red flags

×Only checks bracket counts without checking nesting order (e.g., accepts '([)]' as balanced).
×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 a clear opening-to-closing mapping and a defined stack structure.

Common red flags

×Contains stack underflow risks 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, unmatched opening, unmatched closing, wrong-order nesting, and non-bracket characters.
✓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 linear auxiliary space in the worst case (all openings) and compares them with the repeated-pass baseline.

Staff-level signals

◆Discusses stack depth limits for very long inputs without losing focus.

Common red flags

×Cannot state the complexity of the implemented solution.
×Fails a reasonable follow-up after solving an easy first problem, despite having at least five minutes and receiving bounded clarification.

Trade-offs to articulate

Repeated-removal scanning avoids an explicit stack structure but costs quadratic time in the worst case.
A stack-based single pass achieves linear time using auxiliary space proportional to the maximum nesting depth.

Practice follow-up questions

1.Coding follow-up: given an unbalanced bracket string, compute the minimum number of bracket insertions needed to make it balanced.
2.Verbal extension: how would the approach change if brackets could also be quote-style markers that must not overlap improperly with bracket pairs?

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

StacksStringsComplexity Analysis