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Instacart·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Apr 2026

Summary

Got a coding round at Instacart for a software engineer role and the question was a full arithmetic expression evaluator. More involved than I expected for a phone screen, covering parsing, precedence, unary minus, and error handling all at once.

Questions Asked (1)

Q1

Implement an evaluator for a string arithmetic expression that handles +, -, *, /, parentheses, spaces, operator precedence, left-to-right associativity, unary minus, truncating integer division, and returns a 64-bit integer. Also handle errors like mismatched parens or division by zero. Describe your approach and its complexity.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

I went with recursive descent because I can never remember the two-stack approach under pressure, and it felt more natural to reason about.

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AI HintsAI Generated

Suggested Approach

Use a two-stack approach (operators and operands) or recursive descent parser to evaluate the expression while respecting precedence and associativity. Handle unary minus by treating it as a special operator or by converting to a binary operation with a leading zero. Validate parentheses and division by zero, and use 64-bit integers with truncating division.

Pro tip: Clarify the expected behavior for edge cases like unary minus in different contexts (e.g., '--5' or '5*-3') and integer division truncation (toward zero). Mention that you'd write unit tests for these cases to ensure correctness.

1. Clarify requirements and edge cases

Ask about input constraints, expected output type, handling of unary minus, division truncation, and error reporting. Confirm that spaces should be ignored and that parentheses must be balanced.

2. Choose parsing strategy

Decide between two-stack (shunting-yard) and recursive descent. Explain the trade-offs: two-stack is iterative and straightforward for precedence, while recursive descent is elegant and extensible.

3. Outline algorithm steps

Describe how to process tokens: push operands, compare operator precedence, apply operators, handle parentheses, and manage unary minus. Include error checks for mismatched parentheses and division by zero.

4. Analyze complexity and edge cases

State time complexity O(n) and space O(n) for both approaches. Discuss edge cases like unary minus at start or after operator, nested parentheses, and integer overflow (though 64-bit may suffice).

5. Summarize and offer to code

Concisely recap the approach and complexity, then offer to implement the solution in code if desired, showing readiness to dive deeper.

Key Points to Mention

  • Operator precedence and left-to-right associativity for +, -, *, /
  • Handling unary minus (e.g., -5, 3*-2) by treating as unary operator or converting to binary with 0
  • Truncating integer division toward zero (e.g., 7/2=3, -7/2=-3)
  • Error handling: mismatched parentheses and division by zero
  • Time and space complexity: O(n) time, O(n) space
  • Choice of data structures: two stacks (operators and operands) or recursive descent

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