Introduction to Stacks and Queues
Stacks and Queues are linear data structures with strict rules on how data is added and removed.
- Stack (LIFO): Last-In, First-Out. Think of a stack of plates. You add to the top and take from the top.
- Queue (FIFO): First-In, First-Out. Think of a line at a grocery store. The first person in line is the first one served.
1. Real-World Intuition
- Stack: The "Undo" button in your text editor. Every action is pushed onto a stack. When you hit undo, the last action is the first to be reversed.
- Queue: A printer job queue. Documents are printed in the order they were sent.
2. Curriculum in this Module
- Theory & Intuition (Current Page)
- Problem: Valid Parentheses - The classic stack application.
- Problem: Next Greater Element - Introduction to monotonic logic.
- Lesson: Monotonic Stack Pattern - A deep dive into $O(n)$ neighbor comparisons.
- Curated Practice Problems - 10 essential challenges.
3. Java Implementation Snippets
Stack (using Deque)
Deque<Integer> stack = new ArrayDeque<>();
stack.push(1);
int val = stack.pop();
Queue
Queue<Integer> queue = new LinkedList<>();
queue.offer(1);
int val = queue.poll();
Final Takeaway
Stacks are your primary tool for Nested Logic and Backtracking. Queues are for Order Preservation and Level-order Traversals.