Implementation of Deque using doubly linked list

Difficulty Level : Easy
Last Updated : 14 Jan, 2021

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

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

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

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

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Operations on Deque :

CPP

Java