TL;DR: For LeetCode 224, scan the expression once while maintaining a current result, the number being read, and its sign. When an opening parenthesis appears, save the outer result and sign on a stack; when it closes, finish the inner expression and combine it with that saved context. Time is O(n) and auxiliary space is O(d), where d is nesting depth.
LeetCode 224 Basic Calculator: Stack Solution Explained
Understand the LeetCode 224 Basic Calculator solution with invariants, unary signs, parentheses, complexity, tests, and interview explanation tips.
Try YesToTheOfferWhat is the core idea behind Basic Calculator?
The expression contains nonnegative integers, spaces, plus, minus, and parentheses. Multiplication precedence is absent, but unary signs and nesting create the real challenge. A strong interview solution defines an invariant before coding: result is the completed value in the current parenthesis level, number is the integer currently being parsed, and sign tells how that number contributes.
Start with the job description and mark the skills that appear more than once. Build a small evidence bank from your real work, study, or volunteer experience. Practice a direct answer, then a follow-up that explains your decision, tradeoff, and result. Record a mock session and review where your answer became vague or too long.
How does the stack solution work step by step?
- Ignore spaces without changing state.
- For a digit, extend number as number Ă 10 + digit.
- For plus or minus, add sign Ă number to result, reset number, and set the next sign.
- For an opening parenthesis, save outer result and outer sign, then reset for the inner expression.
- For a closing parenthesis, finish the pending number, multiply the inner result by the saved sign, and add the saved outer result.
- After the scan, add the final pending number.
- Test simple addition, subtraction, spaces, nested groups, and unary minus.
What invariant prevents parser mistakes?
At every operator or closing parenthesis, all digits accumulated in number belong to exactly one term and must be committed once. The stack stores context rather than every token: an outer partial result followed by the sign applied to the parenthesized expression. State the pop order explicitly; reversing it is a common bug. In languages with fixed-width integers, discuss the problemâs numeric guarantees.
| Focus | What to demonstrate | Weak approach |
|---|---|---|
| Digits | Build one multi-digit number | Treat each digit as a separate term |
| Operator | Commit the previous number exactly once | Change sign before committing |
| Parenthesis | Save outer result and sign | Push every character |
| Complexity | One scan; stack grows with nesting | Claim constant space despite nesting |
A practical preparation workflow
Trace 1 - (2 + (3 - 4)) in a table with index, character, result, number, sign, and stack. Then implement from the invariant rather than memorizing code. Test 0, 1 + 1, 2-1 + 2, -(3+4), 1-(-2), and deeply nested input if the stated grammar permits it. Explain how your handling of unary minus follows from the same sign state.
Start with the job description and mark the skills that appear more than once. Build a small evidence bank from your real work, study, or volunteer experience. Practice a direct answer, then a follow-up that explains your decision, tradeoff, and result. Record a mock session and review where your answer became vague or too long.
How should you use interview support responsibly?
Check the employerâs and interview platformâs rules before using any tool in a live assessment. In restricted or proctored sessions, rely on preparation only. When assistance is allowed, keep the final answer in your own words, verify technical suggestions, protect confidential information, and remain ready to explain every claim.
How does YesToTheOffer fit this workflow?
YesToTheOffer can ground preparation in your resume, job description, company notes, and private context. Its desktop workflow supports real-time transcription, answer structuring, coding assistance, and post-interview review. Use those features to organize your own evidence and reasoning, not to invent experience. Learn more through these guides: AI interview copilot, specialized interview support, resume-grounded answer workflow.
Frequently asked questions
FAQ
What is the best approach for LeetCode 224 Basic Calculator?
A single pass with a stack is the standard approach. Track the current result, pending number, and sign; save the outer result and sign when entering parentheses and combine them when leaving.
Why is a stack needed?
Parentheses suspend the outer expression while an inner expression is evaluated. The stack remembers the outer partial result and the sign applied to the group, supporting arbitrary nesting.
What is the time and space complexity?
Each character is processed a constant number of times, so time is O(n). The stack uses O(d) auxiliary space for parenthesis depth, which is O(n) in the worst case.
How do you handle unary minus?
Treat sign as the multiplier for the next number or parenthesized group. With correctly defined state and grammar, a leading minus or a minus after an opening parenthesis uses the same transition.
Should I memorize the Basic Calculator code?
Memorize the invariant and transitions, not exact code. In an interview, tracing one nested example and explaining why each number is committed once is more robust than recalling a snippet.
Turn preparation into a repeatable system
Start with the job description and mark the skills that appear more than once. Build a small evidence bank from your real work, study, or volunteer experience. Practice a direct answer, then a follow-up that explains your decision, tradeoff, and result. Record a mock session and review where your answer became vague or too long.
Turn preparation into a repeatable system
Understand the LeetCode 224 Basic Calculator solution with invariants, unary signs, parentheses, complexity, tests, and interview explanation tips.
Try YesToTheOffer