Meta·Machine Learning Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

May 2026Remote

Summary

Meta MLE coding round focused on a classic expression evaluation problem, but with a twist: they wanted two distinct implementations and a complexity discussion. Not the hardest problem on paper, but the O(1) space variant tripped me up more than I expected.

Questions Asked (1)

Implement a calculator that evaluates an arithmetic expression with non-negative integers, '+' and '*' operators, and optional whitespace. No parentheses. Standard operator precedence applies. Provide both a split-based solution and a single-pass O(1)-space solution, and compare their tradeoffs.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The split approach came naturally: split on '+' to get additive terms, then split each term on '*' and multiply across.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem constraints and edge cases, then present the split-based solution using a stack to handle operator precedence, followed by the single-pass O(1)-space solution that tracks the last term. Compare their time/space complexity, code simplicity, and suitability for different scenarios, emphasizing the tradeoffs.

Pro tip: Mention that the single-pass solution can be extended to handle parentheses with a stack, but the split-based solution would require recursion or a more complex parser. This shows foresight and understanding of real-world parsing challenges.

1. Clarify requirements and edge cases

Ask about input format, possible edge cases (e.g., leading/trailing spaces, multiple digits, zero), and whether the expression is guaranteed valid. Confirm that only '+' and '*' are present and no parentheses.

2. Present split-based solution

Explain splitting the string by '+' to get terms, then for each term split by '*' and multiply the numbers. Sum the results. Use a stack or direct computation. Mention time O(n) and space O(n) due to storing tokens.

3. Present single-pass O(1)-space solution

Describe iterating through the string, maintaining a running sum and a 'last' value for the current term. When encountering '+', add the last term to sum and reset; when '*', multiply last by the next number. At the end, add the last term. Space O(1), time O(n).

4. Compare tradeoffs

Discuss that split-based is simpler to implement and understand but uses extra space; single-pass is more efficient in space but slightly trickier to code. Consider readability, maintainability, and performance for large inputs.

5. Discuss extensions and real-world relevance

Mention how to handle parentheses (stack-based for single-pass, recursion for split-based) and relate to ML engineering tasks like parsing configuration strings or expression evaluation in models.

Key Points to Mention

Operator precedence: multiplication before addition.
Time complexity: both O(n) where n is length of string.
Space complexity: split-based O(n), single-pass O(1).
Edge cases: multiple digits, whitespace, zero, large numbers (overflow).
Code simplicity vs. efficiency tradeoff.
Extensibility to parentheses and other operators.

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