Implementation of Deque using doubly linked list

Deque or Double Ended Queue is a generalized version of Queue data structure that allows insert and delete at both ends. In previous post Implementation of Deque using circular array has been discussed. Now in this post we see how we implement Deque using Doubly Linked List.

Operations on Deque :

Mainly the following four basic operations are performed on queue : 

insertFront() : 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.
size()     : Gets number of elements in Deque.
erase()    : Deletes all the elements from Deque.

Doubly Linked List Representation of Deque :
For implementing deque, we need to keep track of two pointers, front and rear. We enqueue (push) an item at the rear or the front end of deque and dequeue(pop) an item from both rear and front end.

Working :
Declare two pointers front and rear of type Node, where Node represents the structure of a node of a doubly linked list. Initialize both of them with value NULL.

Insertion at Front end :

1. Allocate space for a newNode of doubly linked list.
2. IF newNode == NULL, then
3.     print "Overflow"
4. ELSE
5.     IF front == NULL, then
6.         rear = front = newNode
7.     ELSE
8.         newNode->next = front
9.       front->prev = newNode
10.        front = newNode 

Insertion at Rear end :

1. Allocate space for a newNode of doubly linked list.
2. IF newNode == NULL, then
3.     print "Overflow"
4. ELSE
5.     IF rear == NULL, then
6.         front = rear = newNode
7.     ELSE
8.         newNode->prev = rear
9.       rear->next = newNode
10.        rear = newNode 

Deletion from Front end :

1. IF front == NULL
2.     print "Underflow"
3. ELSE
4.     Initialize temp = front
5.     front = front->next
6.     IF front == NULL
7.         rear = NULL
8.     ELSE
9.         front->prev = NULL
10     Deallocate space for temp

Deletion from Rear end :

1. IF front == NULL
2.     print "Underflow"
3. ELSE
4.     Initialize temp = rear
5.     rear = rear->prev
6.     IF rear == NULL
7.         front = NULL
8.     ELSE
9.         rear->next = NULL
10     Deallocate space for temp

Implementation:

CPP

// C++ implementation of Deque using
// doubly linked list
#include <bits/stdc++.h>

using namespace std;

// Node of a doubly linked list
struct Node {
    int data;
    Node *prev, *next;
    // Function to get a new node
    static Node* getnode(int data)
    {
        Node* newNode = (Node*)malloc(sizeof(Node));
        newNode->data = data;
        newNode->prev = newNode->next = NULL;
        return newNode;
    }
};

// A structure to represent a deque
class Deque {
    Node* front;
    Node* rear;
    int Size;

public:
    Deque()
    {
        front = rear = NULL;
        Size = 0;
    }

    // Operations on Deque
    void insertFront(int data);
    void insertRear(int data);
    void deleteFront();
    void deleteRear();
    int getFront();
    int getRear();
    int size();
    bool isEmpty();
    void erase();
};

// Function to check whether deque
// is empty or not
bool Deque::isEmpty() { return (front == NULL); }

// Function to return the number of
// elements in the deque
int Deque::size() { return Size; }

// Function to insert an element
// at the front end
void Deque::insertFront(int data)
{
    Node* newNode = Node::getnode(data);
    // If true then new element cannot be added
    // and it is an 'Overflow' condition
    if (newNode == NULL)
        cout << "OverFlow\n";
    else {
        // If deque is empty
        if (front == NULL)
            rear = front = newNode;

        // Inserts node at the front end
        else {
            newNode->next = front;
            front->prev = newNode;
            front = newNode;
        }

        // Increments count of elements by 1
        Size++;
    }
}

// Function to insert an element
// at the rear end
void Deque::insertRear(int data)
{
    Node* newNode = Node::getnode(data);
    // If true then new element cannot be added
    // and it is an 'Overflow' condition
    if (newNode == NULL)
        cout << "OverFlow\n";
    else {
        // If deque is empty
        if (rear == NULL)
            front = rear = newNode;

        // Inserts node at the rear end
        else {
            newNode->prev = rear;
            rear->next = newNode;
            rear = newNode;
        }

        Size++;
    }
}

// Function to delete the element
// from the front end
void Deque::deleteFront()
{
    // If deque is empty then
    // 'Underflow' condition
    if (isEmpty())
        cout << "UnderFlow\n";

    // Deletes the node from the front end and makes
    // the adjustment in the links
    else {
        Node* temp = front;
        front = front->next;

        // If only one element was present
        if (front == NULL)
            rear = NULL;
        else
            front->prev = NULL;
        free(temp);

        // Decrements count of elements by 1
        Size--;
    }
}

// Function to delete the element
// from the rear end
void Deque::deleteRear()
{
    // If deque is empty then
    // 'Underflow' condition
    if (isEmpty())
        cout << "UnderFlow\n";

    // Deletes the node from the rear end and makes
    // the adjustment in the links
    else {
        Node* temp = rear;
        rear = rear->prev;

        // If only one element was present
        if (rear == NULL)
            front = NULL;
        else
            rear->next = NULL;
        free(temp);

        // Decrements count of elements by 1
        Size--;
    }
}

// Function to return the element
// at the front end
int Deque::getFront()
{
    // If deque is empty, then returns
    // garbage value
    if (isEmpty())
        return -1;
    return front->data;
}

// Function to return the element
// at the rear end
int Deque::getRear()
{
    // If deque is empty, then returns
    // garbage value
    if (isEmpty())
        return -1;
    return rear->data;
}

// Function to delete all the elements
// from Deque
void Deque::erase()
{
    rear = NULL;
    while (front != NULL) {
        Node* temp = front;
        front = front->next;
        free(temp);
    }
    Size = 0;
}

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

    cout << "Insert element '10' at rear end\n";
    dq.insertRear(10);

    cout << "Rear end element: " << dq.getRear() << endl;

    dq.deleteRear();
    cout << "After deleting rear element new rear"
         << " is: " << dq.getRear() << endl;

    cout << "Inserting element '15' at front end \n";
    dq.insertFront(15);

    cout << "Front end element: " << dq.getFront() << endl;

    cout << "Number of elements in Deque: " << dq.size()
         << endl;

    dq.deleteFront();
    cout << "After deleting front element new "
         << "front is: " << dq.getFront() << endl;

    return 0;
}

Java

// Java implementation of Deque using
// doubly linked list
import java.util.*;
class GFG {

    // Node of a doubly linked list
    static class Node {
        int data;
        Node prev, next;

        // Function to get a new node
        static Node getnode(int data)
        {
            Node newNode = new Node();
            newNode.data = data;
            newNode.prev = newNode.next = null;
            return newNode;
        }
    };

    // A structure to represent a deque
    static class Deque {
        Node front;
        Node rear;
        int Size;

        Deque()
        {
            front = rear = null;
            Size = 0;
        }

        // Function to check whether deque
        // is empty or not
        boolean isEmpty() { return (front == null); }

        // Function to return the number of
        // elements in the deque
        int size() { return Size; }

        // Function to insert an element
        // at the front end
        void insertFront(int data)
        {
            Node newNode = Node.getnode(data);
            // If true then new element cannot be added
            // and it is an 'Overflow' condition
            if (newNode == null)
                System.out.print("OverFlow\n");
            else {
                // If deque is empty
                if (front == null)
                    rear = front = newNode;

                // Inserts node at the front end
                else {
                    newNode.next = front;
                    front.prev = newNode;
                    front = newNode;
                }

                // Increments count of elements by 1
                Size++;
            }
        }

        // Function to insert an element
        // at the rear end
        void insertRear(int data)
        {
            Node newNode = Node.getnode(data);
            // If true then new element cannot be added
            // and it is an 'Overflow' condition
            if (newNode == null)
                System.out.print("OverFlow\n");
            else {
                // If deque is empty
                if (rear == null)
                    front = rear = newNode;

                // Inserts node at the rear end
                else {
                    newNode.prev = rear;
                    rear.next = newNode;
                    rear = newNode;
                }

                Size++;
            }
        }

        // Function to delete the element
        // from the front end
        void deleteFront()
        {
            // If deque is empty then
            // 'Underflow' condition
            if (isEmpty())
                System.out.print("UnderFlow\n");

            // Deletes the node from the front end and makes
            // the adjustment in the links
            else {
                Node temp = front;
                front = front.next;

                // If only one element was present
                if (front == null)
                    rear = null;
                else
                    front.prev = null;

                // Decrements count of elements by 1
                Size--;
            }
        }

        // Function to delete the element
        // from the rear end
        void deleteRear()
        {
            // If deque is empty then
            // 'Underflow' condition
            if (isEmpty())
                System.out.print("UnderFlow\n");

            // Deletes the node from the rear end and makes
            // the adjustment in the links
            else {
                Node temp = rear;
                rear = rear.prev;

                // If only one element was present
                if (rear == null)
                    front = null;
                else
                    rear.next = null;

                // Decrements count of elements by 1
                Size--;
            }
        }

        // Function to return the element
        // at the front end
        int getFront()
        {
            // If deque is empty, then returns
            // garbage value
            if (isEmpty())
                return -1;
            return front.data;
        }

        // Function to return the element
        // at the rear end
        int getRear()
        {

            // If deque is empty, then returns
            // garbage value
            if (isEmpty())
                return -1;
            return rear.data;
        }

        // Function to delete all the elements
        // from Deque
        void erase()
        {
            rear = null;
            while (front != null) {
                Node temp = front;
                front = front.next;
            }
            Size = 0;
        }
    }

    // Driver program to test above
    public static void main(String[] args)
    {
        Deque dq = new Deque();
        System.out.print(
            "Insert element '5' at rear end\n");
        dq.insertRear(5);

        System.out.print(
            "Insert element '10' at rear end\n");
        dq.insertRear(10);
        System.out.print("Rear end element: " + dq.getRear()
                         + "\n");
        dq.deleteRear();
        System.out.print(
            "After deleting rear element new rear"
            + " is: " + dq.getRear() + "\n");
        System.out.print(
            "Inserting element '15' at front end \n");
        dq.insertFront(15);
        System.out.print(
            "Front end element: " + dq.getFront() + "\n");

        System.out.print("Number of elements in Deque: "
                         + dq.size() + "\n");
        dq.deleteFront();
        System.out.print("After deleting front element new "
                         + "front is: " + dq.getFront()
                         + "\n");
    }
}

// This code is contributed by gauravrajput1

Python3

class GFG:
    # Node of a doubly linked list
    class Node:
        data = 0
        prev = None
        next = None

        # Function to get a new node
        @staticmethod
        def getnode(data):
            newNode = GFG.Node()
            newNode.data = data
            newNode.prev = None
            newNode.next = None
            return newNode

    # A structure to represent a deque
    class Deque:
        front = None
        rear = None
        Size = 0

        def __init__(self):
            self.front = None
            self.rear = None
            self.Size = 0

        # Function to check whether deque
        # is empty or not
        def isEmpty(self):
            return (self.front == None)

        # Function to return the number of
        # elements in the deque
        def size(self):
            return self.Size

        # Function to insert an element
        # at the front end
        def insertFront(self, data):
            newNode = GFG.Node.getnode(data)

            # If true then new element cannot be added
            # and it is an 'Overflow' condition
            if (newNode == None):
                print("OverFlow\n", end="")
            else:

                # If deque is empty
                if (self.front == None):
                    self.rear = newNode
                    self.front = newNode
                else:
                    newNode.next = self.front
                    self.front.prev = newNode
                    self.front = newNode

                # Increments count of elements by 1
                self.Size += 1

        # Function to insert an element
        # at the rear end
        def insertRear(self, data):
            newNode = GFG.Node.getnode(data)

            # If true then new element cannot be added
            # and it is an 'Overflow' condition
            if (newNode == None):
                print("OverFlow\n", end="")
            else:

                # If deque is empty
                if (self.rear == None):
                    self.front = newNode
                    self.rear = newNode
                else:
                    newNode.prev = self.rear
                    self.rear.next = newNode
                    self.rear = newNode
                self.Size += 1

        # Function to delete the element
        # from the front end
        def deleteFront(self):

            # If deque is empty then
            # 'Underflow' condition
            if (self.isEmpty()):
                print("UnderFlow\n", end="")
            else:
                temp = self.front
                self.front = self.front.next

                # If only one element was present
                if (self.front == None):
                    self.rear = None
                else:
                    self.front.prev = None

                # Decrements count of elements by 1
                self.Size -= 1

        # Function to delete the element
        # from the rear end
        def deleteRear(self):

            # If deque is empty then
            # 'Underflow' condition
            if (self.isEmpty()):
                print("UnderFlow\n", end="")
            else:
                temp = self.rear
                self.rear = self.rear.prev

                # If only one element was present
                if (self.rear == None):
                    self.front = None
                else:
                    self.rear.next = None

                # Decrements count of elements by 1
                self.Size -= 1

        # Function to return the element
        # at the front end
        def getFront(self):

            # If deque is empty, then returns
            # garbage value
            if (self.isEmpty()):
                return -1
            return self.front.data

        # Function to return the element
        # at the rear end
        def getRear(self):

            # If deque is empty, then returns
            # garbage value
            if (self.isEmpty()):
                return -1
            return self.rear.data

        # Function to delete all the elements
        # from Deque
        def erase(self):
            self.rear = None
            while (self.front != None):
                temp = self.front
                self.front = self.front.next
            self.Size = 0

    # Driver program to test above
    @staticmethod
    def main(args):
        dq = GFG.Deque()
        print("Insert element \'5\' at rear end\n", end="")
        dq.insertRear(5)
        print("Insert element \'10\' at rear end\n", end="")
        dq.insertRear(10)
        print("Rear end element: " + str(dq.getRear()) + "\n", end="")
        dq.deleteRear()
        print("After deleting rear element new rear" +
              " is: " + str(dq.getRear()) + "\n", end="")
        print("Inserting element \'15\' at front end \n", end="")
        dq.insertFront(15)
        print("Front end element: " + str(dq.getFront()) + "\n", end="")
        print("Number of elements in Deque: " + str(dq.size()) + "\n", end="")
        dq.deleteFront()
        print("After deleting front element new " +
              "front is: " + str(dq.getFront()) + "\n", end="")


if __name__ == "__main__":
    GFG.main([])

    # This code is contributed by aadityaburujwale.

C#

using System;

class Deque
{
    // Node of a doubly linked list
    class Node
    {
        public int data;
        public Node prev, next;

        // Function to get a new node
        public static Node GetNode(int data)
        {
            Node newNode = new Node
            {
                data = data,
                prev = null,
                next = null
            };
            return newNode;
        }
    }

    Node front, rear;
    int size;  // Corrected the name to 'size'

    public Deque()
    {
        front = rear = null;
        size = 0;
    }

    // Operations on Deque
    public void InsertFront(int data)
    {
        Node newNode = Node.GetNode(data);
        if (newNode == null)
        {
            Console.WriteLine("OverFlow");
        }
        else
        {
            if (front == null)
                rear = front = newNode;
            else
            {
                newNode.next = front;
                front.prev = newNode;
                front = newNode;
            }
            size++;
        }
    }

    public void InsertRear(int data)
    {
        Node newNode = Node.GetNode(data);
        if (newNode == null)
        {
            Console.WriteLine("OverFlow");
        }
        else
        {
            if (rear == null)
                front = rear = newNode;
            else
            {
                newNode.prev = rear;
                rear.next = newNode;
                rear = newNode;
            }
            size++;
        }
    }

    public void DeleteFront()
    {
        if (IsEmpty())
            Console.WriteLine("UnderFlow");
        else
        {
            Node temp = front;
            front = front.next;
            if (front == null)
                rear = null;
            else
                front.prev = null;
            size--;
            Free(temp);
        }
    }

    public void DeleteRear()
    {
        if (IsEmpty())
            Console.WriteLine("UnderFlow");
        else
        {
            Node temp = rear;
            rear = rear.prev;
            if (rear == null)
                front = null;
            else
                rear.next = null;
            size--;
            Free(temp);
        }
    }

    public int GetFront()
    {
        if (IsEmpty())
            return -1;
        return front.data;
    }

    public int GetRear()
    {
        if (IsEmpty())
            return -1;
        return rear.data;
    }

    public int Size()  // Corrected the name to 'Size'
    {
        return size;
    }

    public bool IsEmpty()
    {
        return (front == null);
    }

    public void Erase()
    {
        rear = null;
        while (front != null)
        {
            Node temp = front;
            front = front.next;
            Free(temp);
        }
        size = 0;
    }

    void Free(Node node)
    {
        // In C#, you generally don't need to manually free memory as the garbage collector handles it.
        // If this were a more complex data structure with other resources, you might need to dispose of them appropriately.
    }
}

class Program
{
    static void Main()
    {
        Deque dq = new Deque();
        Console.WriteLine("Insert element '5' at rear end");
        dq.InsertRear(5);

        Console.WriteLine("Insert element '10' at rear end");
        dq.InsertRear(10);

        Console.WriteLine("Rear end element: " + dq.GetRear());

        dq.DeleteRear();
        Console.WriteLine("After deleting rear element new rear is: " + dq.GetRear());

        Console.WriteLine("Inserting element '15' at front end");
        dq.InsertFront(15);

        Console.WriteLine("Front end element: " + dq.GetFront());

        Console.WriteLine("Number of elements in Deque: " + dq.Size());

        dq.DeleteFront();
        Console.WriteLine("After deleting front element new front is: " + dq.GetFront());
    }
}

Javascript

// Javascript implementation of Deque using
// Node of a doubly linked list
class Node {
    data = 0;
    prev = null;
    next = null;

    // Function to get a new node
    static getnode(data) {
        const newNode = new Node();
        newNode.data = data;
        newNode.prev = null;
        newNode.next = null;
        return newNode;
    }
}

// A structure to represent a deque
class Deque {
    front = null;
    rear = null;
    size = 0;

    constructor() {
        this.front = null;
        this.rear = null;
        this.size = 0;
    }

    // Function to check whether deque
    // is empty or not
    isEmpty() {
        return this.front === null;
    }

    // Function to return the number of
    // elements in the deque
    size() {
        return this.size;
    }

    // Function to insert an element
    // at the front end
    insertFront(data) {
        const newNode = Node.getnode(data);

        // If true then new element cannot be added
        // and it is an 'Overflow' condition
        if (newNode === null) {
            console.log("OverFlow\n");
        } else {
            // If deque is empty
            if (this.front === null) {
                this.rear = newNode;
                this.front = newNode;
            } else {
                newNode.next = this.front;
                this.front.prev = newNode;
                this.front = newNode;
            }

            // Increments count of elements by 1
            this.size += 1;
        }
    }

    // Function to insert an element
    // at the rear end
    insertRear(data) {
        const newNode = Node.getnode(data);

        // If true then new element cannot be added
        // and it is an 'Overflow' condition
        if (newNode === null) {
            console.log("OverFlow\n");
        } else {
            // If deque is empty
            if (this.rear === null) {
                this.front = newNode;
                this.rear = newNode;
            } else {
                newNode.prev = this.rear;
                this.rear.next = newNode;
                this.rear = newNode;
            }
            this.size += 1;
        }
    }

    // Function to delete the element
    // from the front end
    deleteFront() {
        // If deque is empty then
        // 'Underflow' condition
        if (this.isEmpty()) {
            console.log("UnderFlow\n");
        } else {
            const temp = this.front;
            this.front = this.front.next;

            // If only one element was present
            if (this.front === null) {
                this.rear = null;
            } else {
                this.front.prev = null;
            }

            // Decrements count of elements by 1
            this.size -= 1;
        }
    }
    deleteRear() {
        // If deque is empty then
        // 'Underflow' condition
        if (this.isEmpty()) {
            console.log("UnderFlow\n");
        } else {
            const temp = this.rear;
            this.rear = this.rear.prev;

            // If only one element was present
            if (this.rear === null) {
                this.front = null;
            } else {
                this.rear.next = null;
            }

            // Decrements count of elements by 1
            this.size -= 1;
        }
    }

    // Function to return the element
    // at the front end
    getFront() {
        // If deque is empty, then returns
        // garbage value
        if (this.isEmpty()) {
            return -1;
        }
        return this.front.data;
    }

    // Function to return the element
    // at the rear end
    getRear() {
        // If deque is empty, then returns
        // garbage value
        if (this.isEmpty()) {
            return -1;
        }
        return this.rear.data;
    }

    // Function to delete all the elements
    // from Deque
    erase() {
        this.rear = null;
        while (this.front !== null) {
            const temp = this.front;
            this.front = this.front.next;
        }
        this.size = 0;
    }
}



// Driver program to test the Deque class
function main() {
    // Create a Deque object
    const dq = new Deque();

    console.log("Insert element '5' at rear end");
    dq.insertRear(5);

    console.log("Insert element '10' at rear end");
    dq.insertRear(10);

    console.log(`Rear end element: ${dq.getRear()}`);

    dq.deleteRear();
    console.log(`After deleting rear element new rear is: ${dq.getRear()}`);

    console.log("Inserting element '15' at front end");
    dq.insertFront(15);

    console.log(`Front end element: ${dq.getFront()}`);

    console.log(`Number of elements in Deque: ${dq.size}`);

    dq.deleteFront();
    console.log(`After deleting front element new front is: ${dq.getFront()}`);
}

// Call the main function
main();

Output

Insert element '5' at rear end
Insert element '10' at rear end
Rear end element: 10
After deleting rear element new rear is: 5
Inserting element '15' at front end 
Front end element: 15
Number of elements in Deque: 2
After deleting front element new front is: 5

Complexity Analysis:

• Time Complexity : Time complexity of operations like insertFront(), insertRear(), deleteFront(), deleteRear() is O(1). The Time Complexity of erase() is O(n).
• Auxiliary space: O(1)