Instacart·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026Remote

Summary

Instacart coding round, one meaty problem the whole time. The question was about evaluating arithmetic expressions from scratch, which sounds manageable until you're actually writing a recursive parser under pressure.

Questions Asked (1)

Given a string containing an arithmetic expression with non-negative integers, the four basic operators, parentheses, and optional spaces, evaluate it and return the result as a 64-bit integer. Division should truncate toward zero. Your solution should handle operator precedence, unary minus, and be implemented using depth-first traversal (either recursive descent or a stack-based simulation). Also discuss time and space complexity, recursion depth concerns, and how you'd convert to an iterative approach.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

This wrecked me a little.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem constraints and edge cases, then outline a recursive descent parser with a clear grammar. Walk through the algorithm step-by-step, emphasizing how precedence, parentheses, and unary minus are handled. Finally, discuss complexity, recursion depth, and how to convert to an iterative stack-based approach.

Pro tip: Mention that you'd use a stack-based approach in production to avoid stack overflow on deeply nested expressions, and note that integer overflow is possible so you'd use 64-bit integers and consider overflow checks.

1. Clarify requirements and edge cases

Ask about input constraints (e.g., max length, nesting depth), expected behavior for division by zero, and whether unary plus is allowed. Confirm that division truncates toward zero.

2. Define grammar and parsing strategy

Outline a grammar that handles precedence (e.g., expression -> term -> factor) and unary minus. Choose recursive descent for simplicity or a stack-based simulation for iterative robustness.

3. Walk through the algorithm

Describe how the parser processes tokens: skip spaces, parse numbers, handle parentheses recursively, and apply operators with correct precedence. For iterative, explain how to use two stacks (operands and operators).

4. Analyze complexity and trade-offs

State time complexity O(n) and space complexity O(n) for both approaches. Discuss recursion depth concerns (O(n) worst-case) and how iterative avoids stack overflow but uses explicit stack.

5. Discuss conversion to iterative

Explain how to convert recursive descent to an iterative stack-based evaluator: use an operator stack and operand stack, handle precedence by comparing operator priorities, and manage parentheses as markers.

Key Points to Mention

Operator precedence and associativity (left-to-right for same precedence).
Handling unary minus (e.g., -5, -(3+4)) by treating it as a unary operator with high precedence.
Division truncation toward zero (e.g., -7/2 = -3) and potential division by zero.
Time complexity O(n) and space complexity O(n) for both recursive and iterative approaches.
Recursion depth can be O(n) in worst case (e.g., deeply nested parentheses), risking stack overflow.
Iterative approach uses explicit stacks for operands and operators, avoiding recursion limits.

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