The document discusses stacks and queues as data structures. It provides motivation for why data structure choice affects algorithm time complexity. It then covers implementation of stacks and queues using arrays and linked lists, including defining necessary functions like isEmpty, push, pop, and peek. Examples and steps are provided for implementing each with arrays and linked lists. The document also discusses using stacks and queues from the C++ STL and provides a problem to check for palindromes in a string using one pass and a stack or queue.

1. Data Structures
Stack and Queue
Charanjit Ghai
B.Tech , CSE

2. Pre-Requisites
You must know what an array is and how to
access the ith element in an array
You must know what a linked list is and how to
implement the basic operations in a Linked
List

3. Motivation ?
Sometimes the running time of our algorithm
(as in Order notation) greatly varies according
to how we store and retrieve our data.
Example ?
Palindrome Checking using start and end
indices. Time Complexity
string(char array) -> O(n)
Singly Linked List -> O(n^2)

4. What is a Stack ?
What all stuff can we do with this ?
● We can Add one more plate onto
the top.
● We can Remove the top most
plate.
● We can See the top most plate
Ever used a bucket for carrying
clothes for washing ? The cloth you
push at last on the bucket, comes
out first.
A Stack Of Plates

5. Stack in Programming
Similar is the idea of stack in Programming
● You can add an element onto the stack
● You can remove the top most element
● You can see the top most element

6. How to implement a stack then ?
You must have 4 functions for the stack:
isEmpty , Push , Pop and Peek(Top)
isEmpty : Checks if the stack is empty
Push : Push another element onto stack
Pop : Pop the top most element out of stack
Peek : Return the top most element

7. Demo #1
Stack implementation using arrays

8. Approach
Need to have a struct for stack having an array
and top_of_stack variable.
isEmpty : Condition for stack being empty.
Push : Need to check for “over” push.
Pop : Need to check for “over” pop.
Peek : Need to check if stack is empty ?
Confused ? Lets’ go step by step

9. Step #1
Stack Definition ?

10. Step #2
Initialize the stack ?

11. Step #3
isEmpty function ?

12. Step #4
Push Function ?

13. Step #5
Pop Function ?
Note: Can we use any function here ?

14. Step #6
Peek Function ?

15. Phew !! Lets’ Test it now

16. Demo #2
Stack implementation using Linked List

17. Approach
isEmpty : Condition for stack being empty.
Push : No need to check for bounds. Can add
any number of elements until it fits in
memory.
Pop/Peek : Need to check if stack is empty ?

18. Step #1
Stack Definition ?

19. Step #2
Stack Initialization ?

20. Step #3
isEmpty Function

21. Step #4
Push Operation ?
If we insert at the end, then each push operation will take
O(n) time. ouch!!
Solution : We can use the tail pointer as well, and update
it accordingly. Then, push will be O(1).
But, now pop operation will take O(n) time, since for
popping you will need to update tail pointer and we don’t
have prev pointer in node struct.
Solution : Why not insert at head instead. In that case
push (insert at head) , pop (delete head) , and peek (get
head value) will be O(1)). Yess!!

22. Step #4
Push Operation ?
required

23. Step #5
Pop Operation ?

24. Step #6
Peek Operation ?

25. Phew !! Lets’ Test it now

26. What is a Queue ?
What all stuff is possible in the queue ?
● Another person may join the queue
● The person at Front may leave the
queue
● The person at Front may deal with
the person in-charge
Ever stood in a queue for dosa ?
A Queue of People

27. Queue in Programming
Similar is the idea of queue in Programming
● You can add an element at the back
● You can remove an element from the front
● You can see the element in front

28. How to implement a queue then ?
You must have 4 functions for the queue:
isEmpty , Push , Pop and Peek
isEmpty : Check if the queue is empty
Push : Push another element at the back
Pop : Pop out the element at the front
Peek : Return element at front

29. Demo #3
Queue implementation using arrays

30. Approach
Push : Need to check for overflow
Pop/Peek : Need to check if queue is empty ?

31. Step #1
Queue Definition ?

32. Step #2
Queue Initialization ?

33. Step #3
isEmpty Function ?

34. Step #4
Push Operation ?

35. Step #5
Pop Operation ?

36. Step #6
Peek Operation ?

37. Phew !! Lets’ Test it now

38. Limitations and Solutions
Note : once an element is popped, we are
unable to use its’ space.
Solution: Use Circular queue:
front == end -> empty
(end + 1)%size == front -> full
Push -> end = (end + 1)%size
Pop -> front = (front + 1)%size

39. Demo #4
Queue implementation using Linked List

40. Approach
Push : No erroneous case
Pop/Peek : Need to check if queue is empty ?

41. Step #1
Queue Definition ?

42. Step #2
Queue Initialization ?

43. Step #3
isEmpty Function?

44. Step #4
Push Operation ?

45. Step #5
Pop Operation ?

46. Step #6
Peek Operation ?

47. Phew !! Lets’ Test it now

48. Using STL
Using STL stack
Note: Stack is empty while we check top.
You are responsible for your stack.

49. Using STL
Using STL queue
Note: Queue is empty while we check front.
You are responsible for your queue.

50. Problem:
Check if a string is palindrome or not using
only one pass through the string and only one
index variable. You are allowed to use one
stack and one queue.
P.S.: I know the constraints do not make much
sense, but I think we got the same problem in
CS 101 End Sem.

51. Thank You