← Bytedance Interview Insights

Bytedance·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026

Summary

Bytedance SWE coding round, one question the whole time but it had way more surface area than it looked. The validation requirements are what got me, not the arithmetic.

Questions Asked (1)

Q1

Implement a calculator that evaluates an expression string containing only non-negative integers with + and - operators. The input must be validated and rejected if it's empty, has consecutive operators, has leading or trailing operators, contains non-digit characters, or has numbers with leading zeros (except the literal '0'). Discuss edge cases and complexity.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

I jumped straight to the parsing logic and had a working solution pretty fast, then they started poking at edge cases and I slowly realized I'd ignored half the prompt.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and edge cases, then propose a two-phase solution: first validate the input string with a single pass, then evaluate using a running total and sign tracking. Discuss time and space complexity, and mention potential optimizations or alternative approaches.

Pro tip: Explicitly separate validation from evaluation to make the code cleaner and easier to test; also, consider using a state machine for validation to handle all edge cases systematically.

1. Clarify requirements and edge cases

Restate the problem and confirm assumptions: only non-negative integers, + and - operators, no spaces, and specific rejection rules. List all invalid cases to ensure alignment.

2. Design validation logic

Plan a single-pass validation that checks for empty string, leading/trailing operators, consecutive operators, non-digit characters, and leading zeros in numbers (except '0'). Use a state machine or flags to track position and previous token.

3. Design evaluation algorithm

After validation, evaluate the expression by parsing numbers and applying operators. Maintain a running total and a sign variable, updating the total when an operator is encountered.

4. Analyze complexity and edge cases

State that both validation and evaluation are O(n) time and O(1) space. Discuss edge cases like '0', '0+0', '1+2-3', and invalid inputs like '01', '1++2', '+1', '1+', 'a+1'.

5. Consider trade-offs and extensions

Mention potential trade-offs: e.g., combining validation and evaluation for efficiency vs. separation for clarity. Discuss extending to support multiplication/division or parentheses, and how that would change the approach.

Key Points to Mention

  • Input validation rules: empty string, leading/trailing operators, consecutive operators, non-digit characters, leading zeros (except '0').
  • Single-pass validation using a state machine or flags to track previous character type.
  • Evaluation using a running total and sign variable, updating on operators.
  • Time complexity O(n) and space complexity O(1) for both validation and evaluation.
  • Edge cases: '0', '0+0', '1+2-3', '01', '1++2', '+1', '1+', 'a+1'.
  • Trade-offs: combining validation and evaluation vs. separating them; extensibility to more operators or parentheses.

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