DS UNIT 2 PPT.pptx stack queue representations

SHUNMUGAVADIVOO.T,
ASST PROFESSOR,
DEPARTMENT OF COMPUTER SCIENCE,
S.A.COLLEGE OF ARTS & SCIENCE,
CHENNAI – 600077
TAMILNADU
DATA STRUCTURE – STACK AND QUEUE

The stack abstract data type is defined by the following structure and
operations. A stack is structured, as described above, as an ordered collection of
items where items are added to and removed from the end called the “top.”
Stacks are ordered LIFO.
Primitive Stack Operations
Stack operations may involve initializing the stack, using it and then de-
initializing it. Apart from these basic stuffs, a stack is used for the following two
primary operations −
push() − Pushing (storing) an element on the stack.
pop() − Removing (accessing) an element from the stack.
3
THE STACK ABSTRACT
DATA TYPE

PRIMITIVE STACK
OPERATIONS
The following diagram depicts a stack and its operations −
4

PUSH
OPERATION
The process of putting a new data element onto stack is known as a Push Operation.
Push operation involves a series of steps −
•
•
•
•
•
Step 1 − Checks if the stack is full.
Step 2 − If the stack is full, produces an error and exit.
Step 3 − If the stack is not full, increments top to point next empty space.
Step 4 − Adds data element to the stack location, where top is pointing.
Step 5 − Returns success.
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POP
OPERATION
A Pop operation may involve the following steps −
•
•
•
•
•
Step 1 − Checks if the stack is empty.
Step 2 − If the stack is empty, produces an error and exit.
Step 3 − If the stack is not empty, accesses the data element at which top is
pointing.
Step 4 − Decreases the value of top by 1.
Step 5 − Returns success.
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Algorithm of Push and Pop Operation
7
PUS H
begin procedure push: stack,
data
if stack is full
return null
endif top ← top + 1
stack[top] ← data
end procedure
Pop
begin procedure pop: stack
if stack is empty
return null
endif data ← stack[top]
top ← top – 1
return data
end procedure

ARRAY
REPRESENTATION OF
STACK
Stack can be represented by means of a one way list or a linear array.
A pointer variable top contains the locations of the top element of
the stack and a variable max stk gives the maximum number of
elements of the Stack that can be held by Stack.
the condition top=0 will indicate that the stack is empty.
This procedure pushes an item onto the Stack via Top.
PUSH(Stack, Top, MaxStk, Item)
1. If Top = = MaxStk //check Stack already fill or not
then print "Overflow" and return
2. Set Top = Top + 1 //increase top by 1
3.Stack[Top] = Item //insert item in new top position 4. Return
Let MaxStk=8, the array Stack contains M, N, O in it. Perform
operations on it
8

ARRAY REPRESENTATION OF
STACK
Pop operation on Stack
This procedures deletes the top element of Stack and assigns
it to the variable item.
POP(Stack, top, Item)
1.If Top = = Null //check Stack top element to be deleted is
empty
then print "Underflow" and return
2. Item = Stack[Top] //assign top element to item
3. Set Top = Top - 1 //decrease top by 1
4. Return
pop 3 elements
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REPRESENTATION OF
STACK
• Each node in the stack should contain two parts:
– info: the user's data
– next: the address of the next element in the stack
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Node Type
template<class ItemType>
struct NodeType {
ItemType info;
NodeType* next;
};

LINKED
REPRESENTATION OF
STACK
First and last stack elements
We need a data member to store the pointer to the top of the
stack
The next element of the last node should contain the
value NULL
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APPLICATIONS OF
STACKS
1)Reversing Strings:
• A simple application of stack is reversing strings.To reverse a
string , the characters of string are pushed onto the stack one by one
as the string
is read from left to right.
•Once all the characters of string are pushed onto stack, they are
popped one by one. Since the character last pushed in comes out
first, subsequent pop
operation results in the reversal of the string.
For example:
To reverse the string ‘REVERSE’ the string is
read from left to right and its characters are
pushed . LIKE:
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APPLICATIONS OF
STACKS
2)Evaluating arithmetic expressions:
• INFIX notation:
The general way of writing arithmetic
expressions is known as infix notation.
e.g, (a+b)
• PREFIX notation:
e.g, + A B
• POSTFIX notation:
e.g: A B +
Conversion of INFIX to POSTFIX conversion:
Example: 2+(4-1)*3 step1
2+41-*3 step2
2+41-3*
step3 241-
3*+ step4
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CONVERSION OF INFIX INTO
POSTFIX 2+(4-1)*3 INTO
241-3*+
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APPLICATIONS OF
STACKS
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3) CONVERSION OF INFIX INTO POSTFIX EXPRESSION
•
•
•
•
•
Output operands as encountered
Stack left parentheses
When ‘)’
– repeat
• pop stack, output
symbol
– until ‘(‘
•‘(‘ is poped but not output
If symbol +, *, or ‘(‘
– pop stack until entry of
lower priority or ‘(‘
• ‘(‘ removed only when
matching ‘)’ is processed
– push symbol into stack
At end of input, pop stack until
empty

ALGORITHM TO EVALUATE A
POSTFIX EXPRESSION
16
•
•
•
Use a stack, assume binary operators +,*
Input: postfix expression
Scan the input
– If operand,
• push to stack
– If operator
• pop the stack twice
• apply operator
• push result back to stack

QUEUE
17
Stores a set of elements in a particular
order Queue principle: FIRST IN
FIRST OUT= FIFO
It means: the first element inserted is the first one
to be removed
Example
The first one in line is the first one to be served.

FIRST IN
FIRST OUT
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rear
front
C
B
A
rear
front
D
C
B
A
rear
front

ARRAY-BASED QUEUE
IMPLEMENTATION
• As with the array-based stack
implementation, the array is of fixed size
–A queue of maximum N elements
Slightly more complicated
– Need to maintain track of both
front and rear
•
Implementation 1
Implementation 2
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IMPLEMENTATION
enqueue
void enqueue(int *rear,
element item)
{
/* add an item to the queue */
if (*rear = =
MAX_QUEUE_SIZE_1) {
queue_full( );
return;
}
queue [++*rear]
= item;
}
Dequeue
element dequeue(int *front,
int rear)
{
/* remove element at the
front of the queue */
if ( *front = = rear)
return
queue_empty( );
/* return an error key */
return queue [ + + *front];
}
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IMPLEMENTING QUEUES USING
LINKED LISTS
21
•
•
•
Allocate memory for each new element
dynamically
Link the queue elements together
Use two pointers, qFront and qRear, to mark the
front and rear of the queue

TOWER OF HANOI
22
•
•
•
•
•
•
•
There are three towers
3 gold disks, with decreasing sizes, placed on the first
tower
You need to move all of the disks from the first tower
to the last tower
Larger disks can not be placed on top of smaller disks
The third tower can be used to temporarily hold disks
The disks must be moved within one week. Assume
one disk can be moved in 1 second. Is this possible?
To create an algorithm to solve this problem, it is
convenient to generalize the problem to the “N-disk”
problem, where in our case N = 3.

Leave one empty space when queue is full
Why?
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ABSTRACT DATA TYPE:
PRIORITY QUEUE
• A priority queue is a collection of zero or more items,
– associated with each item is a priority
• A priority queue has at least three operations
– insert(item i) (enqueue) a new item
– delete() (dequeue) the member with the highest
priority
– find() the item with the highest priority
– decreasePriority(item i, p) decrease the priority
of ith item to p
• Note that in a priority queue "first in first out" does not apply in general.
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PRIORITY QUEUES:
ASSUMPTIONS
• The highest priority can be either the minimum
value of all the items, or the maximum.
– We will assume the highest priority is the
minimum.
– Call the delete operation deleteMin().
– Call the find operation findMin().
• Assume the priority queue has n members
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DS UNIT 2 PPT.pptx stack queue representations

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