This content originally appeared on DEV Community and was authored by M Umair Ullah
Mastering Stack Data Structure – From Basics to LeetCode Practice
Stacks are one of the most fundamental data structures in computer science and play a critical role in problem-solving, algorithms, and real-world systems. In this post, I’ll walk you through what a stack is, why we need it, important algorithms, and how I’m practicing LeetCode stack problems to master this concept. I’ll also share my GitHub repository link at the end where I upload all my practice codes.
What is a Stack?
A stack is a linear data structure that follows the principle of LIFO (Last In, First Out).
- Imagine stacking plates: you can only take out the plate on the top.
- The same rule applies to a stack: Insertions (push) and deletions (pop) happen only at one end (top).
Basic Operations:
-
push(x): Insert element
xat the top. - pop(): Remove the top element.
- top()/peek(): Get the top element without removing it.
- isEmpty(): Check if stack is empty.
Why Do We Need a Stack?
Stacks may sound simple, but they are powerful and used everywhere:
- Expression Evaluation: Balanced Parentheses, Infix → Postfix conversion.
- Backtracking: Undo/redo in editors, recursion call stack.
- Monotonic Stack Problems: Next Greater Element, Stock Span, Daily Temperatures.
- System Design: Function calls in memory are handled using a call stack.
Important Stack Problem References
Here are the must-practice problems (with links) that cover all the essential stack patterns:
Easy
- LeetCode 20 – Valid Parentheses
- LeetCode 155 – Min Stack
- LeetCode 225 – Implement Stack using Queues
Medium
- LeetCode 496 – Next Greater Element I
- LeetCode 503 – Next Greater Element II
- LeetCode 739 – Daily Temperatures
- LeetCode 901 – Online Stock Span
Hard
Time & Space Complexity Analysis
| Operation | Time Complexity | Space Complexity |
|---|---|---|
| Push | O(1) | O(1) |
| Pop | O(1) | O(1) |
| Peek/Top | O(1) | O(1) |
| Search | O(n) | O(1) |
| Next Greater Element / Daily Temperatures | O(n) | O(n) |
Most stack-based problems (like NGE, Daily Temperatures) are O(n) because each element is pushed and popped at most once.
My LeetCode Practice Plan
To fully master stacks, I’m solving these problems step by step:
- Easy Level: Warm up with Valid Parentheses, Min Stack.
- Medium Level: Focus on monotonic stack problems (NGE, Daily Temperatures, Stock Span).
- Hard Level: Move on to Histogram and Matrix-based problems.
This ensures I cover all core algorithms involving stacks.
GitHub Repository
I’m uploading all my stack solutions (brute force + optimized stack approach) to my GitHub repo.
Check it out here: My GitHub Repo – Stack Practice
Conclusion
Stacks are simple but powerful. By practicing problems like Next Greater Element, Daily Temperatures, Histogram problems, you’ll not only master the stack but also build a strong foundation for advanced algorithms and interview prep.
My approach: I’m practicing every problem type, pushing solutions to GitHub, and writing blogs to keep track of my journey.
If you’re also practicing, try out these problems, optimize your solutions, and let’s grow together!
This content originally appeared on DEV Community and was authored by M Umair Ullah