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Instacart·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Coding screen for a software engineer role at Instacart. One problem the whole time, string decoding with a stack, but the follow-ups kept coming and that's where things got interesting.

Questions Asked (1)

Q1

Given a nested encoded string where digits followed by parentheses mean 'repeat the contents k times', decode it manually using a stack. No regex allowed.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

The base case was fine.

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AI HintsAI Generated

Suggested Approach

Clarify the problem and constraints, then walk through a stack-based solution step by step, explaining how to handle digits, letters, and brackets. Emphasize the role of the stack in managing nested repetitions and maintaining the current string and repeat count.

Pro tip: Mention that you can optimize space by using a single stack that stores both the repeat count and the string built so far, and discuss how this approach avoids regex and handles arbitrary nesting.

1. Clarify and Define

Ask clarifying questions about input format, constraints (e.g., digits can be multi-digit, brackets are balanced), and expected output. Define the problem precisely.

2. Outline Stack Approach

Explain that you'll use a stack to store pairs of (previous string, repeat count) when encountering '['. Maintain a current string and current number.

3. Walk Through Example

Trace the algorithm on a simple nested example like '3[a2[c]]' to demonstrate how the stack unwinds and builds the result.

4. Discuss Complexity and Edge Cases

Analyze time and space complexity (O(n) time, O(n) space). Mention edge cases: multi-digit numbers, empty brackets, no nesting, and invalid input.

5. Code and Test

Write clean code with meaningful variable names, then test with provided examples and edge cases. Explain any trade-offs if using a different data structure.

Key Points to Mention

  • Use a stack to handle nested repetitions by saving the current string and repeat count when encountering '['.
  • Parse multi-digit numbers correctly by accumulating digits until a non-digit is encountered.
  • When encountering ']', pop the previous string and repeat count, then append the repeated current string to the previous string.
  • Time complexity is O(n) where n is the length of the decoded string, and space complexity is O(n) for the stack and output.
  • Avoid regex as instructed; instead, manually parse characters and use the stack for control flow.
  • Consider edge cases like empty input, no brackets, and nested brackets with multiple digits.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.