1. Data Structures
Lakshmish Ramaswamy

2. Stack Data Structure Last-in-first-out paradigm
Supports two operations
Push – Insert an element at top
Pop – Remove the element at top
Can be implemented using ArrayLists or LinkedLists
How?

3. Queue First-in-First-Out Paradigm Many applications Operations
Buffer management
Job schedulers in OS
Operations
enqueue
Dequeue
getFront
Can be implemented using ArrayList or LinkedList

4. Queue

5. Implementation Using ArrayList/LinkedList
enqueue(o) implemented as add(o)
Added to the end
dequeue() implemented as remove(0)
getFirst() implemented as get(0)
Problem with ArrayList implementation
Every dequeue causes element shifting

6. Circular Queue Avoids element shifting on enqueue or dequeue
Circle with numbered slots
Slot numbers increase in clockwise fashion
“Capacity” indicates maximum elements queue can hold
Two pointers – Front and End
-1 indicates empty queue
Both pointers move in clockwise direction
Wraparound on reaching end

7. Circular Queue Illustration
Front
C-1 1 2
End

8. Circular Queue Implementation
public class cirQueue{
Object[] arr;
int capacity;
int front;
int end;
public cirQueue(int cpty){
capacity = cpty;
arr = new Object[capacity];
front = -1;
end = -1;}

9. Enqueue Method public boolean enqueue(Object o){ if(end == -1){
front = 0;
arr[end] = o;
return(true);}
int newEnd = (end + 1)%capacity;
if (newEnd == front) return(false);
end = newEnd;
return(true); }

10. Dequeue Method public Object dequeue(){ Object retobj;
if(front == -1) return(null);
retobj = arr[front];
if(front == end){
front = -1;
end = -1;}
else{
front = (front+1)%capacity;}
return(retobj);}