Implement Stack with O(n) time
Last updated: January 4, 2026
Quick Overview
Implement a stack data structure that supports push, pop, and top operations, ensuring that all operations run in O(n) time. The stack should maintain the order of elements and allow for retrieval of the top element without removing it. Your implementation should handle edge cases such as underflow when popping from an empty stack.
Dropbox
January 4, 2026212
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3,156 solved
Implement a stack data structure that supports push, pop, and top operations, ensuring that all operations run in O(n) time. The stack should maintain the order of elements and allow for retrieval of the top element without removing it. Your implementation should handle edge cases such as underflow when popping from an empty stack.
This coding problem is frequently asked during Phone Screen at Dropbox. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Dropbox expects candidates to write production-quality code, not just solve the puzzle.
What the Interviewer Expects
- Identify the correct data structure and algorithm for the problem
- Write clean, bug-free code with proper variable naming
- Analyze time and space complexity correctly
- Handle basic edge cases (empty input, single element)
- Communicate your thought process while coding
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- How would you parallelize this solution?
- Can you solve this in a single pass?
- What happens if the input contains duplicates?
- Can you optimize the space complexity of your solution?
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Practice DSA ProblemsSample Answer
Problem Analysis
To implement a stack with O(n) time complexity for push, pop, and top operations, we need to understand that while typical stack operations are O(1), the constraint here is to ensure all operations ma...
Approach
- Initialize two lists:
stackto hold the elements andorderto keep track of the order of elements. - For the
pushoperation, append the new element to thestackand to theorderlist as ...
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