Bytedance·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Bytedance software engineer round that was basically one big two-part coding problem: validate an arithmetic expression, then evaluate it. The kind of question that feels manageable until you start edge-casing it to death.

Questions Asked (2)

Given an input string, determine whether it is a syntactically valid arithmetic expression. The string may contain digits, the operators +, -, *, /, spaces, and parentheses. You need to handle things like balanced parentheses, no adjacent operators, and no leading or trailing operators.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I thought I had this.

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AI HintsAI Generated

Suggested Approach

Clarify the exact grammar rules (e.g., unary operators, decimal points) and then propose a linear-time parser using a state machine or stack to track parentheses and operator placement. Discuss trade-offs between regex, recursive descent, and iterative parsing, emphasizing efficiency and edge-case handling.

Pro tip: Mention that a stack-based approach can validate parentheses and operator placement in one pass, but be prepared to discuss how to extend it for unary operators or implicit multiplication. Also, explicitly state assumptions about the input format to avoid ambiguity.

1. Clarify Requirements and Edge Cases

Ask about allowed characters, unary operators (e.g., negative numbers), decimal points, and whether empty strings or spaces-only are valid. Confirm that the expression must be fully parsed with no leftover characters.

2. Define a Grammar or State Machine

Outline a formal grammar or state transitions (e.g., expecting operand, operator, or parenthesis) to systematically validate the string. This helps in explaining the logic clearly.

3. Choose an Algorithm and Data Structure

Select an approach: stack for parentheses and operator tracking, or a simple state variable for linear scan. Discuss time and space complexity (O(n) time, O(1) or O(n) space).

4. Walk Through Examples and Edge Cases

Test the algorithm on cases like '1+2*3', '(1+2)*3', '1++2', '()', '1+', and '((1+2)'. Explain how each is handled to demonstrate correctness.

5. Discuss Extensions and Trade-offs

Mention how to extend for unary minus, decimals, or variables, and compare with alternative approaches like regex or recursive descent. Highlight why your chosen method is optimal for the given constraints.

Key Points to Mention

Balanced parentheses using a counter or stack
No adjacent operators (except unary minus if allowed)
No leading or trailing operators
Handling of spaces (ignore or treat as delimiters)
Time and space complexity: O(n) time, O(1) or O(n) space
Edge cases: empty string, single number, nested parentheses

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

Now evaluate the valid arithmetic expression and return the result. Operator precedence applies: multiplication and division before addition and subtraction, with parentheses overriding that order. Handle negative numbers.

Algorithms & Data Structures

Author's notes

This is basically the classic stack-based calculator problem and I knew it, which helped.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints (e.g., integer division, whitespace, unary minus) and then propose a two-stack or shunting-yard algorithm to handle operator precedence and parentheses. Walk through the algorithm step-by-step, emphasizing how to manage negative numbers and edge cases, and analyze time/space complexity.

Pro tip: Mention that you would first convert the expression to Reverse Polish Notation (RPN) using the shunting-yard algorithm, then evaluate it with a stack—this cleanly separates parsing from evaluation and handles precedence and parentheses elegantly. Also, proactively discuss how to handle unary minus (e.g., by treating it as part of the number or using a special operator) to show attention to detail.

1. Clarify requirements and constraints

Ask about input format (spaces, valid characters), integer vs floating-point division, handling of unary minus, and expected output type. Confirm that the expression is always valid.

2. Choose an algorithm

Select a two-stack approach (operators and operands) or the shunting-yard algorithm to convert to RPN and then evaluate. Explain why this handles precedence and parentheses.

3. Handle negative numbers and unary operators

Describe how to distinguish unary minus from binary subtraction (e.g., based on context: start of expression, after '(' or another operator). Propose a strategy such as treating '-5' as a single token or using a special marker.

4. Walk through the algorithm with an example

Trace the algorithm on a sample expression like '3 - 4 * (2 + (-5))' to demonstrate correctness, showing stack operations and operator precedence handling.

5. Analyze complexity and edge cases

State time and space complexity (O(n) for both). Discuss edge cases: nested parentheses, division by zero, large numbers, and multiple unary operators.

Key Points to Mention

Operator precedence and associativity rules (left-to-right for +,-,*,/).
Use of stacks for operands and operators or shunting-yard for RPN conversion.
Handling parentheses by evaluating sub-expressions when a closing parenthesis is encountered.
Distinguishing unary minus from binary subtraction based on context.
Time and space complexity: O(n) time, O(n) space in worst case.
Edge cases: division by zero, integer overflow, and whitespace handling.

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