1. Lecture 21
Stacks and Queues
Richard Gesick

2. Stacks Implement a first-in-last-out behavior Push() to add an element
Pop() to remove an element
Add and remove from the same side of the internal data structure
Easiest to add/remove from the front if implemented as a linked list internally
Peek() returns the top element without removing it
from the stack

3. Stacks C# stack is generic. Consider:
Stack <string> myStack= new Stack>string>();
myStack.Push(“hello”);
myStack.Push(“fun”);
myStack.Push(“great”);
Stack state?
string t=myStack.Pop();
string s= myStack.Peek();
What is stored in s, t and what is left in the stack?

4. Stacks
Which data structure would you pick to implement a Stack? Why?

5. Queues Implement a first-in-first-out behavior
Enqueue() to add an element
Dequeue() to remove an element
Add and remove from the opposite sides of the
internal data structure
If we maintain a list-tail reference, then we can
add to the end and remove from the front,
each having constant - O(1) - time

6. Queues C# Queue is generic. Consider: State of the queue?
Queue<int> q= new Queue<int>();
q.Enqueue(5);
q.Enqueue(7);
q.Enqueue(1);
State of the queue?
int a= q.Dequeue();
int b= q.Dequeue();
What is stored in a, b and what left in the queue?

7. Queues
Which data structure would you pick to implement a Queue? Why?