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Instacart·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Coding round at Instacart for a software engineer role. Two meaty implementation questions back to back, both with a lot of edge case discussion baked in. Felt more like a design conversation than a pure coding session.

Questions Asked (2)

Q1

Build an arithmetic expression evaluator that reads expressions from a file, one per line, and prints each result. It needs to handle integers, the four basic operators, parentheses, operator precedence, and whitespace. Walk through how you'd deal with invalid tokens, divide-by-zero, overflow, and very long lines, plus the data structures and complexity behind your approach.

Algorithms & Data StructuresTechnical Trade-offsSystem Design
Author's notes

This one went longer than I expected.

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AI HintsAI Generated

Suggested Approach

Start by outlining a two-phase approach: tokenization (lexer) and parsing (e.g., recursive descent or shunting-yard) to handle precedence and parentheses. Then systematically address each edge case (invalid tokens, divide-by-zero, overflow, long lines) with specific strategies, and conclude with data structures and complexity analysis.

Pro tip: Mention that you'd use a streaming line reader to handle very long lines without loading the entire file into memory, and discuss how you'd extend the evaluator to support additional operators or functions—showing foresight beyond the immediate requirements.

1. Clarify requirements and constraints

Confirm the expected input format, integer size (e.g., 32-bit vs 64-bit), and whether expressions can contain unary operators or only binary. Ask about error handling expectations (e.g., skip line vs abort).

2. Design the evaluation algorithm

Choose between recursive descent parser or shunting-yard algorithm to convert to RPN and evaluate. Explain how operator precedence and parentheses are handled, and justify your choice based on simplicity and extensibility.

3. Address edge cases and error handling

Detail how you'd detect and handle invalid tokens (e.g., unexpected characters), divide-by-zero (report error and continue), overflow (use big integers or detect and report), and very long lines (stream processing or dynamic buffer).

4. Discuss data structures and complexity

Describe the data structures used (e.g., stacks for operators/operands, token queue) and analyze time and space complexity per expression and overall for the file.

5. Summarize and extend

Recap the approach, mention potential optimizations (e.g., caching, parallel processing), and note how the design could be extended for more operators or functions.

Key Points to Mention

  • Tokenization: separating numbers, operators, and parentheses while ignoring whitespace.
  • Parsing algorithm: recursive descent or shunting-yard for operator precedence and parentheses.
  • Error handling: invalid tokens (report and skip line), divide-by-zero (catch and report), overflow (use big integers or detect).
  • Long lines: streaming line reader or dynamic buffer to avoid memory issues.
  • Data structures: stacks for operators and operands, token queue.
  • Complexity: O(n) time per expression, O(n) space for tokens and stacks; overall O(total input size).

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

Q2

Implement a decoder for strings encoded in the format k[encoded_string], where brackets can be nested (for example, 3[a2[c]] decodes to accaccacc). Handle multi-digit repeat counts, UTF-8 characters, and malformed input gracefully. Aim for O(n) time and space, and discuss both iterative and recursive implementations.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

Classic problem but the follow-ups made it harder than it sounds.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases (nested brackets, multi-digit counts, UTF-8, malformed input), then present a stack-based iterative solution that builds strings and repeat counts, and finally discuss a recursive alternative with trade-offs. Emphasize O(n) time and space by processing each character once and using a stack of frames.

Pro tip: Demonstrate production maturity by explicitly defining malformed-input behavior (e.g., return error or best-effort decode) and by noting UTF-8 handling requires iterating over runes/code points, not bytes, to avoid splitting multibyte characters.

1. Clarify requirements and edge cases

Ask about malformed input handling (throw vs. best-effort), multi-digit counts, UTF-8 characters, and whether the output should be a string or stream. Confirm O(n) time/space expectations.

2. Design stack-based iterative solution

Use a stack of frames (current string, repeat count). Iterate over characters: build numbers, push on '[', pop and repeat on ']', append otherwise. Handle UTF-8 by iterating over code points.

3. Address malformed input gracefully

Define behavior for unmatched brackets, missing counts, or invalid characters—e.g., return an error or decode as much as possible—and implement validation without breaking O(n).

4. Present recursive alternative and trade-offs

Describe a recursive descent parser that processes a segment and returns the decoded string and next index. Compare readability vs. stack depth risk and iterative control over memory.

5. Analyze complexity and test

Explain O(n) time (each character processed once) and O(n) space (stack depth and output size). Walk through examples like '3[a2[c]]' and edge cases (empty string, '10[ab]', UTF-8).

Key Points to Mention

  • Stack-based iterative parsing with frames for string and repeat count
  • Multi-digit number parsing (accumulate digits until '[')
  • UTF-8 handling by iterating over code points/runes, not bytes
  • Malformed input strategy: validation, error handling, or best-effort decoding
  • Recursive implementation trade-offs: simplicity vs. stack overflow risk
  • O(n) time and space analysis, including output size and stack depth

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