# Implementation of Deque using circular array

Deque or Double Ended Queue is a generalized version of Queue data structure that allows insert and delete at both ends.In previous post we had discussed introduction of deque. Now in this post we see how we implement deque Using circular array.
Operations on Deque:
Mainly the following four basic operations are performed on queue:
insetFront(): Adds an item at the front of Deque.
insertRear(): Adds an item at the rear of Deque.
deleteFront(): Deletes an item from front of Deque.
deleteRear(): Deletes an item from rear of Deque.

In addition to above operations, following operations are also supported
getFront(): Gets the front item from queue.
getRear(): Gets the last item from queue.
isEmpty(): Checks whether Deque is empty or not.
isFull(): Checks whether Deque is full or not.

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Circular array implementation deque
For implementing deque, we need to keep track of two indices, front and rear. We enqueue(push) an item at the rear or the front end of qedue and dequeue(pop) an item from both rear and front end.
Working
1. Create an empty array ‘arr’ of size ‘n’
initialize front = -1 , rear = 0
Inserting First element in deque either front end or read end they both lead to the same result.

After insert Front Points = 0 and Rear points = 0
Insert Elements at Rear end

```a). First we check deque if Full or Not
b). IF Rear == Size-1
then reinitialize Rear = 0 ;
Else increment Rear by '1'
and push current key into Arr[ rear ] = key
Front remain same.
```

Insert Elements at Front end

```a). First we check deque if Full or Not
b). IF Front == 0 || initial position, move Front
to points last index of array
front = size - 1
Else decremented front by '1' and push
current key into Arr[ Front] = key
Rear remain same.
```

Delete Element From Rear end

```a). first Check deque is Empty or Not
b).  If deque has only one element
front = -1 ; rear =-1 ;
Else IF Rear points to the first index of array
it's means we have to move rear to points
last index [ now first inserted element at
front end become rear end ]
rear = size-1 ;
Else || decrease rear by '1'
rear = rear-1;
```

Delete Element From Front end

```a). first Check deque is Empty or Not
b).  If deque has only one element
front = -1 ; rear =-1 ;
Else IF front points to the last index of the array
it's means we have no more elements in array so
we move front to points first index of array
front = 0 ;
Else || increment Front by '1'
front = front+1;
```

Below c++ implementation of above idea.

## C++

```// C++ implementation of De-queue using circular
// array
#include<iostream>
using namespace std;

// Maximum size of array or Dequeue
#define MAX 100

// A structure to represent a Deque
class Deque
{
int  arr[MAX];
int  front;
int  rear;
int  size;
public :
Deque(int size)
{
front = -1;
rear = 0;
this->size = size;
}

// Operations on Deque:
void  insertfront(int key);
void  insertrear(int key);
void  deletefront();
void  deleterear();
bool  isFull();
bool  isEmpty();
int  getFront();
int  getRear();
};

// Checks whether Deque is full or not.
bool Deque::isFull()
{
return ((front == 0 && rear == size-1)||
front == rear+1);
}

// Checks whether Deque is empty or not.
bool Deque::isEmpty ()
{
return (front == -1);
}

// Inserts an element at front
void Deque::insertfront(int key)
{
// check whether Deque if  full or not
if (isFull())
{
cout << "Overflow\n" << endl;
return;
}

// If queue is initially empty
if (front == -1)
{
front = 0;
rear = 0;
}

// front is at first position of queue
else if (front == 0)
front = size - 1 ;

else // decrement front end by '1'
front = front-1;

// insert current element into Deque
arr[front] = key ;
}

// function to inset element at rear end
// of Deque.
void Deque ::insertrear(int key)
{
if (isFull())
{
cout << " Overflow\n " << endl;
return;
}

// If queue is initially empty
if (front == -1)
{
front = 0;
rear = 0;
}

// rear is at last position of queue
else if (rear == size-1)
rear = 0;

// increment rear end by '1'
else
rear = rear+1;

// insert current element into Deque
arr[rear] = key ;
}

// Deletes element at front end of Deque
void Deque ::deletefront()
{
// check whether Deque is empty or not
if (isEmpty())
{
cout << "Queue Underflow\n" << endl;
return ;
}

// Deque has only one element
if (front == rear)
{
front = -1;
rear = -1;
}
else
// back to initial position
if (front == size -1)
front = 0;

else // increment front by '1' to remove current
// front value from Deque
front = front+1;
}

// Delete element at rear end of Deque
void Deque::deleterear()
{
if (isEmpty())
{
cout << " Underflow\n" << endl ;
return ;
}

// Deque has only one element
if (front == rear)
{
front = -1;
rear = -1;
}
else if (rear == 0)
rear = size-1;
else
rear = rear-1;
}

// Returns front element of Deque
int Deque::getFront()
{
// check whether Deque is empty or not
if (isEmpty())
{
cout << " Underflow\n" << endl;
return -1 ;
}
return arr[front];
}

// function return rear element of Deque
int Deque::getRear()
{
// check whether Deque is empty or not
if(isEmpty() || rear < 0)
{
cout << " Underflow\n" << endl;
return -1 ;
}
return arr[rear];
}

// Driver program to test above function
int main()
{
Deque dq(5);
cout << "Insert element at rear end  : 5 \n";
dq.insertrear(5);

cout << "insert element at rear end : 10 \n";
dq.insertrear(10);

cout << "get rear element " << " "
<< dq.getRear() << endl;

dq.deleterear();
cout << "After delete rear element new rear"
<< " become " << dq.getRear() << endl;

cout << "inserting element at front end \n";
dq.insertfront(15);

cout << "get front element " << " "
<< dq.getFront() << endl;

dq.deletefront();

cout << "After delete front element new "
<< "front become " << dq.getFront() << endl;
return 0;
}
```

## Java

```// Java implementation of De-queue using circular
// array

// A structure to represent a Deque
class Deque
{
static final int MAX = 100;
int  arr[];
int  front;
int  rear;
int  size;

public Deque(int size)
{
arr = new int[MAX];
front = -1;
rear = 0;
this.size = size;
}

/*// Operations on Deque:
void  insertfront(int key);
void  insertrear(int key);
void  deletefront();
void  deleterear();
bool  isFull();
bool  isEmpty();
int  getFront();
int  getRear();*/

// Checks whether Deque is full or not.
boolean isFull()
{
return ((front == 0 && rear == size-1)||
front == rear+1);
}

// Checks whether Deque is empty or not.
boolean isEmpty ()
{
return (front == -1);
}

// Inserts an element at front
void insertfront(int key)
{
// check whether Deque if  full or not
if (isFull())
{
System.out.println("Overflow");
return;
}

// If queue is initially empty
if (front == -1)
{
front = 0;
rear = 0;
}

// front is at first position of queue
else if (front == 0)
front = size - 1 ;

else // decrement front end by '1'
front = front-1;

// insert current element into Deque
arr[front] = key ;
}

// function to inset element at rear end
// of Deque.
void insertrear(int key)
{
if (isFull())
{
System.out.println(" Overflow ");
return;
}

// If queue is initially empty
if (front == -1)
{
front = 0;
rear = 0;
}

// rear is at last position of queue
else if (rear == size-1)
rear = 0;

// increment rear end by '1'
else
rear = rear+1;

// insert current element into Deque
arr[rear] = key ;
}

// Deletes element at front end of Deque
void deletefront()
{
// check whether Deque is empty or not
if (isEmpty())
{
System.out.println("Queue Underflow\n");
return ;
}

// Deque has only one element
if (front == rear)
{
front = -1;
rear = -1;
}
else
// back to initial position
if (front == size -1)
front = 0;

else // increment front by '1' to remove current
// front value from Deque
front = front+1;
}

// Delete element at rear end of Deque
void deleterear()
{
if (isEmpty())
{
System.out.println(" Underflow");
return ;
}

// Deque has only one element
if (front == rear)
{
front = -1;
rear = -1;
}
else if (rear == 0)
rear = size-1;
else
rear = rear-1;
}

// Returns front element of Deque
int getFront()
{
// check whether Deque is empty or not
if (isEmpty())
{
System.out.println(" Underflow");
return -1 ;
}
return arr[front];
}

// function return rear element of Deque
int getRear()
{
// check whether Deque is empty or not
if(isEmpty() || rear < 0)
{
System.out.println(" Underflow\n");
return -1 ;
}
return arr[rear];
}

// Driver program to test above function
public static void main(String[] args)
{

Deque dq = new Deque(5);

System.out.println("Insert element at rear end  : 5 ");
dq.insertrear(5);

System.out.println("insert element at rear end : 10 ");
dq.insertrear(10);

System.out.println("get rear element : "+ dq.getRear());

dq.deleterear();
System.out.println("After delete rear element new rear become : " +
dq.getRear());

System.out.println("inserting element at front end");
dq.insertfront(15);

System.out.println("get front element: " +dq.getFront());

dq.deletefront();

System.out.println("After delete front element new front become : " +
+  dq.getFront());

}
}
```

Output:

```
insert element at rear end  : 5
insert element at rear end : 10
get rear element : 10
After delete rear element new rear become : 5
inserting element at front end
get front element : 15
After delete front element new front become : 5
```

Time Complexity: Time complexity of all operations like insertfront(), insertlast(), deletefront(), deletelast()is O(1).

In mext post we will discuss deque implementation using Doubly linked list.

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