A stack is a data structure that follows the last-in, first-out (LIFO) principle. Items are added to the top of the stack using a push operation and removed from the top using a pop operation. Stacks can be implemented using either an array or a linked list. With an array, the top index must be tracked, while with a linked list, the top is the first node in the list. Both implementations allow push and pop operations to occur in constant time.

1. Stacks

2. Stacks
 A stack is a sequence of items that are
accessible at only one end of the sequence.
Stacks 2

3. Pushing/Popping a Stack
 Because a pop removes the item last added to
the stack, we say that a stack has LIFO (last-in/
first-out) ordering.
Stacks 3

4. TOP – THE MOST RECENTLY INSERTED
ITEM
PUSH - TO INSERT ONTO THE TOP OF A
STACK
POP - TO REMOVE THE TOP ITEM IN A
STACK
Stacks 4

5. Stack Implementations
Stacks 5
 Array based
 Where is top?
 How are elements added, removed?
 Linked List based
 Where is top?
 How are elements added, removed?
 Efficiency of operations

6. Array Based Stack Implementation
Where should top be?
Stacks 6
12
15
5
2
3
4
stack
[0] array
What are array values?
s.push (20)
s.pop( )

7. Linked List Implementation
Stacks 7
D
C
B
t o p
A
S t a c k
D C B A
L i n k e d L i s t
f r o n t
s.push (‘F’)
s.pop( )

8. Stack with a Singly Linked List
 We can implement a stack with a singly linked list
 The top element is stored at the first node of the list
 The space used is O(n) and each operation of the Stack ADT
takes O(1) time
t Æ
Stacks 8
nodes
elements

9. Thank You

10. Stacks 10