Instacart·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Instacart coding screen for a software engineer role. The problem looked deceptively simple at first glance but there's a lot of edge-case territory once you start thinking about variable resolution order.

Questions Asked (1)

Build an expression evaluator that processes a sequence of variable assignment statements. Each variable can be assigned a literal number, another variable, or a simple arithmetic expression (add, subtract, multiply, divide) using variables and numbers. After all statements are processed, return the resolved value of each variable.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

My first instinct was to just parse line by line and store values in a map, which works fine until you realize variables can reference other variables and you need to trace back through the chain.

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AI HintsAI Generated

Suggested Approach

Model the problem as a directed graph where variables are nodes and assignments define edges, then use topological sort or DFS with memoization to resolve dependencies and detect cycles. Handle arithmetic operations by recursively evaluating operands and applying the operator, ensuring division by zero and undefined variables are addressed.

Pro tip: Clarify upfront whether assignments are sequential (so later assignments can override earlier ones) or if all statements are independent; this affects whether you need to process in order or can treat it as a pure dependency graph. Also, mention that you'd cache evaluated results to avoid redundant computation.

1. Clarify requirements and edge cases

Ask about assignment order, variable redefinition, division by zero, undefined variables, and whether expressions can be nested. Confirm the expected output format (e.g., map of variable to value).

2. Choose a data structure and parsing strategy

Decide how to store assignments (e.g., map from variable to expression AST or string) and how to parse expressions (e.g., recursive descent, shunting-yard, or simple split for binary ops).

3. Design the evaluation algorithm

Use DFS with memoization to evaluate each variable, detecting cycles via a visiting set. For each expression, recursively evaluate operands and apply the operator.

4. Handle errors and edge cases

Define behavior for cycles (throw error or return null), division by zero, and undefined variables. Ensure the algorithm gracefully reports these.

5. Analyze complexity and discuss trade-offs

Explain time complexity O(V+E) for graph traversal and space O(V). Discuss iterative vs recursive approaches and potential optimizations like caching.

Key Points to Mention

Dependency graph representation and cycle detection using DFS or topological sort
Memoization to avoid recomputing variable values
Parsing arithmetic expressions with operator precedence (if needed)
Handling division by zero and undefined variables gracefully
Time and space complexity analysis
Trade-offs between recursive and iterative evaluation, and between eager vs lazy evaluation

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