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Mastering Stacks and Queues: Essential Data Structures

Learn about stacks and queues, their implementation, operations, applications in various scenarios like recursion, maze solving, and more. Dive deep into the fundamentals of these crucial data structures.

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Mastering Stacks and Queues: Essential Data Structures

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  1. Stacks and Queues 15-121 Introduction to Data Structures Ananda Gunawardena

  2. Stack and Heap • A stack is a first-in-last-out structure • When a method is called computer stores all related data in a place called stack • When the method returns, stack is used generate the return parameters

  3. Basic Architecture • Stacks retain • Information • About where to return • Stacks can be used (implicitly) to implement a technique called recursion

  4. A Stack interface public interface Stack { public void push(Object x); public void pop(); public Object top(); public boolean isEmpty(); public void clear(); }

  5. Stacks are LIFO Push operations: e d c b a

  6. Stacks are LIFO Pop operation: e Last element that was pushed is the first to be popped. d c b a

  7. Queues • Behavior • add item only from the back (enqueue) • remove items only from the front (dequeue) • Many Applications • printer queue • network queues • common resource sharing • Queue Operations • enqueue • dequeue • clear • empty • full

  8. A Queue interface public interface Queue { public void enqueue(Object x); public Object dequeue(); public boolean isEmpty(); public void clear(); }

  9. Queues are FIFO back front k r q c m

  10. Queues are FIFO Enqueue operation: back front k r q c m y

  11. Queues are FIFO Enqueue operation: back front y k r q c m

  12. Queues are FIFO Dequeue operation: back front y k r q c m

  13. Implementation

  14. a b c Implementing stacks, 1 Linked representation. All operations constant time, or O(1).

  15. Implementing stacks, 2 • An alternative is to use an array-based representation. • What are some advantages and disadvantages of an array-based representation? a b c top

  16. A queue from two stacks Enqueue: Dequeue: j a What happens when the stack on the right becomes empty? i b h c g d f e

  17. Implementing a Queue with an array

  18. Applications

  19. Applications of Stacks • Balancing Symbols • Problem: Write a program that will check the validity of a statement • Eg: (2 + 3 ( 4 – 5 )) is valid • (2 + 3 (4 – 5 ) is not valid • Discuss an algorithm using stacks

  20. Applications of Stacks ctd.. • Evaluating Postfix expressions • Consider the expression ( 2+ 3*(5 – 2) + 3*2) • How do we write a program to find the answer to the above expression? • Write the expression in postfix notation 2 3 5 2 - * + 3 2 * + • Use a stack to evaluate it.

  21. a b c d e f g h i j k l m n o p Stack Application - Maze • Think about a grid of rooms separated by walls. • Each room can be given a name. Find a path thru the maze from a to p.

  22. Applications of Queue • Breadth First Traversal

  23. Applications of Queue • Snake Game

  24. Homework • Read notes on recursion • Think of more applications where a stack or queue can be used • Read the assignment on Mazes

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