Doubly Linked List | Set 1 (Introduction and Insertion)

24 Jun, 2022
We strongly recommend to refer following post as a prerequisite of this post.Linked List Introduction Inserting a node in Singly Linked ListA Doubly Linked List (DLL) contains an extra pointer, typically called previous pointer, together with next pointer and data which are there in singly linked list. 
Following is representation of a DLL node in C language.
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/* Node of a doubly linked list */class Node {     public:    int data;     Node* next; // Pointer to next node in DLL    Node* prev; // Pointer to previous node in DLL};   // This code is contributed by shivanisinghss2110
/* Node of a doubly linked list */struct Node {    int data;    struct Node* next; // Pointer to next node in DLL    struct Node* prev; // Pointer to previous node in DLL};
// Class for Doubly Linked Listpublic class DLL {    Node head; // head of list      /* Doubly Linked list Node*/    class Node {        int data;        Node prev;        Node next;          // Constructor to create a new node        // next and prev is by default initialized as null        Node(int d) { data = d; }    }}
# Node of a doubly linked list class Node:    def __init__(self, next=None, prev=None, data=None):        self.next = next # reference to next node in DLL        self.prev = prev # reference to previous node in DLL        self.data = data
// Class for Doubly Linked Listpublic class DLL {    Node head; // head of list      /* Doubly Linked list Node*/    public class Node {        public int data;        public Node prev;        public Node next;          // Constructor to create a new node        // next and prev is by default initialized as null        Node(int d) { data = d; }    }}  // This code contributed by gauravrajput1
<script>// Class for Doubly Linked List    var head; // head of list      /* Doubly Linked list Node */     class Node {        // Constructor to create a new node            // next and prev is by default initialized as null            constructor(val) {                this.data = val;                this.prev = null;                this.next = null;            }        }            // This code contributed by gauravrajput1 </script>
Following are advantages/disadvantages of doubly linked list over singly linked list.Advantages over singly linked list 1) A DLL can be traversed in both forward and backward direction. 2) The delete operation in DLL is more efficient if pointer to the node to be deleted is given. 3) We can quickly insert a new node before a given node. In singly linked list, to delete a node, pointer to the previous node is needed. To get this previous node, sometimes the list is traversed. In DLL, we can get the previous node using previous pointer.  
Disadvantages over singly linked list 1) Every node of DLL Require extra space for an previous pointer. It is possible to implement DLL with single pointer though (See this and this). 2) All operations require an extra pointer previous to be maintained. For example, in insertion, we need to modify previous pointers together with next pointers. For example in following functions for insertions at different positions, we need 1 or 2 extra steps to set previous pointer.Insertion A node can be added in four ways 1) At the front of the DLL 2) After a given node. 3) At the end of the DLL 4) Before a given node. 
1) Add a node at the front: (A 5 steps process) The new node is always added before the head of the given Linked List. And newly added node becomes the new head of DLL. For example if the given Linked List is 10152025 and we add an item 5 at the front, then the Linked List becomes 510152025. Let us call the function that adds at the front of the list is push(). The push() must receive a pointer to the head pointer, because push must change the head pointer to point to the new node (See this) 
Following are the 5 steps to add node at the front.
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/* Given a reference (pointer to pointer) to the head of a list and an int, inserts a new node on the front of the list. */void push(Node** head_ref, int new_data) {     /* 1. allocate node */    Node* new_node = new Node();       /* 2. put in the data */    new_node->data = new_data;       /* 3. Make next of new node as head     and previous as NULL */    new_node->next = (*head_ref);     new_node->prev = NULL;       /* 4. change prev of head node to new node */    if ((*head_ref) != NULL)         (*head_ref)->prev = new_node;               /* 5. move the head to point to the new node */    (*head_ref) = new_node; }           // This code is contributed by shivanisinghss2110
/* Given a reference (pointer to pointer) to the head of a list   and an int, inserts a new node on the front of the list. */void push(struct Node** head_ref, int new_data){    /* 1. allocate node */    struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));      /* 2. put in the data  */    new_node->data = new_data;      /* 3. Make next of new node as head and previous as NULL */    new_node->next = (*head_ref);    new_node->prev = NULL;      /* 4. change prev of head node to new node */    if ((*head_ref) != NULL)        (*head_ref)->prev = new_node;      /* 5. move the head to point to the new node */    (*head_ref) = new_node;}
// Adding a node at the front of the listpublic void push(int new_data){    /* 1. allocate node     * 2. put in the data */    Node new_Node = new Node(new_data);      /* 3. Make next of new node as head and previous as NULL */    new_Node.next = head;    new_Node.prev = null;      /* 4. change prev of head node to new node */    if (head != null)        head.prev = new_Node;      /* 5. move the head to point to the new node */    head = new_Node;}
# Adding a node at the front of the listdef push(self, new_data):      # 1 & 2: Allocate the Node & Put in the data    new_node = Node(data = new_data)      # 3. Make next of new node as head and previous as NULL    new_node.next = self.head    new_node.prev = None      # 4. change prev of head node to new node     if self.head is not None:        self.head.prev = new_node      # 5. move the head to point to the new node    self.head = new_node   # This code is contributed by jatinreaper
// Adding a node at the front of the listpublic void push(int new_data){        /* 1. allocate node     * 2. put in the data */    Node new_Node = new Node(new_data);      /* 3. Make next of new node as head and previous as NULL */    new_Node.next = head;    new_Node.prev = null;      /* 4. change prev of head node to new node */    if (head != null)        head.prev = new_Node;      /* 5. move the head to point to the new node */    head = new_Node;}  // This code is contributed by aashish1995 
// Adding a node at the front of the listfunction push(new_data){    /* 1. allocate node     * 2. put in the data */    let new_Node = new Node(new_data);      /* 3. Make next of new node as head and previous as NULL */    new_Node.next = head;    new_Node.prev = null;      /* 4. change prev of head node to new node */    if (head != null)        head.prev = new_Node;      /* 5. move the head to point to the new node */    head = new_Node;}  // This code is contributed by saurabh_jaiswal.
Four steps of the above five steps are same as the 4 steps used for inserting at the front in singly linked list. The only extra step is to change previous of head.2) Add a node after a given node.: (A 7 steps process) We are given pointer to a node as prev_node, and the new node is inserted after the given node. 
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/* Given a node as prev_node, insert a new node after the given node */void insertAfter(Node* prev_node, int new_data) {     /*1. check if the given prev_node is NULL */    if (prev_node == NULL)     {         cout<<"the given previous node cannot be NULL";         return;     }       /* 2. allocate new node */    Node* new_node = new Node();      /* 3. put in the data */    new_node->data = new_data;       /* 4. Make next of new node as next of prev_node */    new_node->next = prev_node->next;       /* 5. Make the next of prev_node as new_node */    prev_node->next = new_node;       /* 6. Make prev_node as previous of new_node */    new_node->prev = prev_node;       /* 7. Change previous of new_node's next node */    if (new_node->next != NULL)         new_node->next->prev = new_node; }   // This code is contributed by shivanisinghss2110.
/* Given a node as prev_node, insert a new node after the given node */void insertAfter(struct Node* prev_node, int new_data){    /*1. check if the given prev_node is NULL */    if (prev_node == NULL) {        printf("the given previous node cannot be NULL");        return;    }      /* 2. allocate new node */    struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));      /* 3. put in the data  */    new_node->data = new_data;      /* 4. Make next of new node as next of prev_node */    new_node->next = prev_node->next;      /* 5. Make the next of prev_node as new_node */    prev_node->next = new_node;      /* 6. Make prev_node as previous of new_node */    new_node->prev = prev_node;      /* 7. Change previous of new_node's next node */    if (new_node->next != NULL)        new_node->next->prev = new_node;}
/* Given a node as prev_node, insert a new node after the given node */public void InsertAfter(Node prev_Node, int new_data){      /*1. check if the given prev_node is NULL */    if (prev_Node == null) {        System.out.println("The given previous node cannot be NULL ");        return;    }      /* 2. allocate node     * 3. put in the data */    Node new_node = new Node(new_data);      /* 4. Make next of new node as next of prev_node */    new_node.next = prev_Node.next;      /* 5. Make the next of prev_node as new_node */    prev_Node.next = new_node;      /* 6. Make prev_node as previous of new_node */    new_node.prev = prev_Node;      /* 7. Change previous of new_node's next node */    if (new_node.next != null)        new_node.next.prev = new_node;}
# Given a node as prev_node, insert# a new node after the given node  def insertAfter(self, prev_node, new_data):          # 1. check if the given prev_node is NULL        if prev_node is None:            print("This node doesn't exist in DLL")            return          #2. allocate node  & 3. put in the data        new_node = Node(data = new_data)          # 4. Make next of new node as next of prev_node        new_node.next = prev_node.next          # 5. Make the next of prev_node as new_node         prev_node.next = new_node          # 6. Make prev_node as previous of new_node        new_node.prev = prev_node          # 7. Change previous of new_node's next node */        if new_node.next is not None:            new_node.next.prev = new_node  #  This code is contributed by jatinreaper
/* Given a node as prev_node, insert a new node after the given node */public void InsertAfter(Node prev_Node, int new_data){      /*1. check if the given prev_node is NULL */    if (prev_Node == null) {        Console.WriteLine("The given previous node cannot be NULL ");        return;    }      /* 2. allocate node     * 3. put in the data */    Node new_node = new Node(new_data);      /* 4. Make next of new node as next of prev_node */    new_node.next = prev_Node.next;      /* 5. Make the next of prev_node as new_node */    prev_Node.next = new_node;      /* 6. Make prev_node as previous of new_node */    new_node.prev = prev_Node;      /* 7. Change previous of new_node's next node */    if (new_node.next != null)        new_node.next.prev = new_node;}  // This code is contributed by aashish1995
<script>  function InsertAfter(prev_Node,new_data){        /*1. check if the given prev_node is NULL */    if (prev_Node == null) {        document.write("The given previous node cannot be NULL <br>");        return;    }      /* 2. allocate node     * 3. put in the data */    let new_node = new Node(new_data);      /* 4. Make next of new node as next of prev_node */    new_node.next = prev_Node.next;      /* 5. Make the next of prev_node as new_node */    prev_Node.next = new_node;      /* 6. Make prev_node as previous of new_node */    new_node.prev = prev_Node;      /* 7. Change previous of new_node's next node */    if (new_node.next != null)        new_node.next.prev = new_node;}    // This code is contributed by unknown2108  </script>
Five of the above steps step process are same as the 5 steps used for inserting after a given node in singly linked list. The two extra steps are needed to change previous pointer of new node and previous pointer of new node’s next node.3) Add a node at the end: (7 steps process) The new node is always added after the last node of the given Linked List. For example if the given DLL is 510152025 and we add an item 30 at the end, then the DLL becomes 51015202530. Since a Linked List is typically represented by the head of it, we have to traverse the list till end and then change the next of last node to new node. 
Following are the 7 steps to add node at the end. 
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/* Given a reference (pointer to pointer) to the head of a DLL and an int, appends a new node at the end */void append(Node** head_ref, int new_data) {     /* 1. allocate node */    Node* new_node = new Node();       Node* last = *head_ref; /* used in step 5*/      /* 2. put in the data */    new_node->data = new_data;       /* 3. This new node is going to be the last node, so         make next of it as NULL*/    new_node->next = NULL;       /* 4. If the Linked List is empty, then make the new         node as head */    if (*head_ref == NULL)    {         new_node->prev = NULL;         *head_ref = new_node;         return;     }       /* 5. Else traverse till the last node */    while (last->next != NULL)         last = last->next;       /* 6. Change the next of last node */    last->next = new_node;       /* 7. Make last node as previous of new node */    new_node->prev = last;       return; }   // This code is contributed by shivanisinghss2110
/* Given a reference (pointer to pointer) to the head   of a DLL and an int, appends a new node at the end  */void append(struct Node** head_ref, int new_data){    /* 1. allocate node */    struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));      struct Node* last = *head_ref; /* used in step 5*/      /* 2. put in the data  */    new_node->data = new_data;      /* 3. This new node is going to be the last node, so          make next of it as NULL*/    new_node->next = NULL;      /* 4. If the Linked List is empty, then make the new          node as head */    if (*head_ref == NULL) {        new_node->prev = NULL;        *head_ref = new_node;        return;    }      /* 5. Else traverse till the last node */    while (last->next != NULL)        last = last->next;      /* 6. Change the next of last node */    last->next = new_node;      /* 7. Make last node as previous of new node */    new_node->prev = last;      return;}
// Add a node at the end of the listvoid append(int new_data){    /* 1. allocate node      * 2. put in the data */    Node new_node = new Node(new_data);      Node last = head; /* used in step 5*/      /* 3. This new node is going to be the last node, so     * make next of it as NULL*/    new_node.next = null;      /* 4. If the Linked List is empty, then make the new     * node as head */    if (head == null) {        new_node.prev = null;        head = new_node;        return;    }      /* 5. Else traverse till the last node */    while (last.next != null)        last = last.next;      /* 6. Change the next of last node */    last.next = new_node;      /* 7. Make last node as previous of new node */    new_node.prev = last;}
# Add a node at the end of the DLLdef append(self, new_data):          # 1. allocate node 2. put in the data        new_node = Node(data = new_data)        last = self.head          # 3. This new node is going to be the         # last node, so make next of it as NULL        new_node.next = None          # 4. If the Linked List is empty, then        #  make the new node as head        if self.head is None:            new_node.prev = None            self.head = new_node            return          # 5. Else traverse till the last node         while (last.next is not None):            last = last.next          # 6. Change the next of last node         last.next = new_node        # 7. Make last node as previous of new node */        new_node.prev = last  #  This code is contributed by jatinreaper
// Add a node at the end of the list     void append(int new_data)     {         /* 1. allocate node         * 2. put in the data */        Node new_node = new Node(new_data);           Node last = head; /* used in step 5*/          /* 3. This new node is going             to be the last node, so         * make next of it as NULL*/        new_node.next = null;                   /* 4. If the Linked List is empty,         then make the new * node as head */        if (head == null)         {             new_node.prev = null;             head = new_node;             return;         }           /* 5. Else traverse till the last node */        while (last.next != null)             last = last.next;           /* 6. Change the next of last node */        last.next = new_node;           /* 7. Make last node as previous of new node */        new_node.prev = last;     }   // This code is contributed by shivanisinghss2110
<script>// Add a node at the end of the listfunction append(new_data){    /* 1. allocate node      * 2. put in the data */    var new_node = new Node(new_data);      var last = head; /* used in step 5*/      /* 3. This new node is going to be the last node, so     * make next of it as NULL*/    new_node.next = null;      /* 4. If the Linked List is empty, then make the new     * node as head */    if (head == null) {        new_node.prev = null;        head = new_node;        return;    }      /* 5. Else traverse till the last node */    while (last.next != null)        last = last.next;      /* 6. Change the next of last node */    last.next = new_node;      /* 7. Make last node as previous of new node */    new_node.prev = last;}  // This code is contributed by Rajput-Ji </script>
Six of the above 7 steps are same as the 6 steps used for inserting after a given node in singly linked list. The one extra step is needed to change previous pointer of new node.4) Add a node before a given node:  
Steps Let the pointer to this given node be next_node and the data of the new node to be added as new_data.  
Check if the next_node is NULL or not. If it’s NULL, return from the function because any new node can not be added before a NULLAllocate memory for the new node, let it be called new_nodeSet new_node->data = new_dataSet the previous pointer of this new_node as the previous node of the next_node, new_node->prev = next_node->prevSet the previous pointer of the next_node as the new_node, next_node->prev = new_nodeSet the next pointer of this new_node as the next_node, new_node->next = next_node;If the previous node of the new_node is not NULL, then set the next pointer of this previous node as new_node, new_node->prev->next = new_nodeElse, if the prev of new_node is NULL, it will be the new head node. So, make (*head_ref) = new_node.
Check if the next_node is NULL or not. If it’s NULL, return from the function because any new node can not be added before a NULL
Allocate memory for the new node, let it be called new_node
Set new_node->data = new_data
Set the previous pointer of this new_node as the previous node of the next_node, new_node->prev = next_node->prev
Set the previous pointer of the next_node as the new_node, next_node->prev = new_node
Set the next pointer of this new_node as the next_node, new_node->next = next_node;
If the previous node of the new_node is not NULL, then set the next pointer of this previous node as new_node, new_node->prev->next = new_node
Else, if the prev of new_node is NULL, it will be the new head node. So, make (*head_ref) = new_node.
 
Below is the implementation of the above approach: 
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Output: 
Created DLL is: 
Traversal in forward Direction
9 1 5 7 6  
Traversal in reverse direction
6 7 5 1 9 
A complete working program to test above functions. Following is complete program to test above functions.  
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// A complete working C++ program to // demonstrate all insertion methods #include <bits/stdc++.h>using namespace std;  // A linked list node class Node {     public:    int data;     Node* next;     Node* prev; };   /* Given a reference (pointer to pointer) to the head of a list and an int, inserts a new node on the front of the list. */void push(Node** head_ref, int new_data) {     /* 1. allocate node */    Node* new_node = new Node();       /* 2. put in the data */    new_node->data = new_data;       /* 3. Make next of new node as head     and previous as NULL */    new_node->next = (*head_ref);     new_node->prev = NULL;       /* 4. change prev of head node to new node */    if ((*head_ref) != NULL)         (*head_ref)->prev = new_node;       /* 5. move the head to point to the new node */    (*head_ref) = new_node; }   /* Given a node as prev_node, insert a new node after the given node */void insertAfter(Node* prev_node, int new_data) {     /*1. check if the given prev_node is NULL */    if (prev_node == NULL)     {         cout<<"the given previous node cannot be NULL";         return;     }       /* 2. allocate new node */    Node* new_node = new Node();      /* 3. put in the data */    new_node->data = new_data;       /* 4. Make next of new node as next of prev_node */    new_node->next = prev_node->next;       /* 5. Make the next of prev_node as new_node */    prev_node->next = new_node;       /* 6. Make prev_node as previous of new_node */    new_node->prev = prev_node;       /* 7. Change previous of new_node's next node */    if (new_node->next != NULL)         new_node->next->prev = new_node; }   /* Given a reference (pointer to pointer) to the head of a DLL and an int, appends a new node at the end */void append(Node** head_ref, int new_data) {     /* 1. allocate node */    Node* new_node = new Node();       Node* last = *head_ref; /* used in step 5*/      /* 2. put in the data */    new_node->data = new_data;       /* 3. This new node is going to be the last node, so         make next of it as NULL*/    new_node->next = NULL;       /* 4. If the Linked List is empty, then make the new         node as head */    if (*head_ref == NULL)    {         new_node->prev = NULL;         *head_ref = new_node;         return;     }       /* 5. Else traverse till the last node */    while (last->next != NULL)         last = last->next;       /* 6. Change the next of last node */    last->next = new_node;       /* 7. Make last node as previous of new node */    new_node->prev = last;       return; }   // This function prints contents of // linked list starting from the given node void printList(Node* node) {     Node* last;     cout<<"\nTraversal in forward direction \n";     while (node != NULL)     {         cout<<" "<<node->data<<" ";         last = node;         node = node->next;     }       cout<<"\nTraversal in reverse direction \n";     while (last != NULL)     {         cout<<" "<<last->data<<" ";         last = last->prev;     } }   /* Driver program to test above functions*/int main() {     /* Start with the empty list */    Node* head = NULL;       // Insert 6. So linked list becomes 6->NULL     append(&head, 6);       // Insert 7 at the beginning. So     // linked list becomes 7->6->NULL     push(&head, 7);       // Insert 1 at the beginning. So     // linked list becomes 1->7->6->NULL     push(&head, 1);       // Insert 4 at the end. So linked     // list becomes 1->7->6->4->NULL     append(&head, 4);       // Insert 8, after 7. So linked     // list becomes 1->7->8->6->4->NULL     insertAfter(head->next, 8);       cout << "Created DLL is: ";     printList(head);       return 0; }   // This is code is contributed by rathbhupendra
// A complete working C program to // demonstrate all insertion// methods#include <stdio.h>#include <stdlib.h>  // A linked list nodestruct Node {    int data;    struct Node* next;    struct Node* prev;};  /* Given a reference (pointer to pointer) to the head of a   list and an int, inserts a new node on the front of the   list. */void push(struct Node** head_ref, int new_data){    /* 1. allocate node */    struct Node* new_node        = (struct Node*)malloc(sizeof(struct Node));      /* 2. put in the data  */    new_node->data = new_data;      /* 3. Make next of new node as head and previous as NULL     */    new_node->next = (*head_ref);    new_node->prev = NULL;      /* 4. change prev of head node to new node */    if ((*head_ref) != NULL)        (*head_ref)->prev = new_node;      /* 5. move the head to point to the new node */    (*head_ref) = new_node;}  /* Given a node as prev_node, insert a new node after the * given node */void insertAfter(struct Node* prev_node, int new_data){    /*1. check if the given prev_node is NULL */    if (prev_node == NULL) {        printf("the given previous node cannot be NULL");        return;    }      /* 2. allocate new node */    struct Node* new_node        = (struct Node*)malloc(sizeof(struct Node));      /* 3. put in the data  */    new_node->data = new_data;      /* 4. Make next of new node as next of prev_node */    new_node->next = prev_node->next;      /* 5. Make the next of prev_node as new_node */    prev_node->next = new_node;      /* 6. Make prev_node as previous of new_node */    new_node->prev = prev_node;      /* 7. Change previous of new_node's next node */    if (new_node->next != NULL)        new_node->next->prev = new_node;}  /* Given a reference (pointer to pointer) to the head   of a DLL and an int, appends a new node at the end  */void append(struct Node** head_ref, int new_data){    /* 1. allocate node */    struct Node* new_node        = (struct Node*)malloc(sizeof(struct Node));      struct Node* last = *head_ref; /* used in step 5*/      /* 2. put in the data  */    new_node->data = new_data;      /* 3. This new node is going to be the last node, so          make next of it as NULL*/    new_node->next = NULL;      /* 4. If the Linked List is empty, then make the new          node as head */    if (*head_ref == NULL) {        new_node->prev = NULL;        *head_ref = new_node;        return;    }      /* 5. Else traverse till the last node */    while (last->next != NULL)        last = last->next;      /* 6. Change the next of last node */    last->next = new_node;      /* 7. Make last node as previous of new node */    new_node->prev = last;      return;}  // This function prints contents of linked list starting// from the given nodevoid printList(struct Node* node){    struct Node* last;    printf("\nTraversal in forward direction \n");    while (node != NULL) {        printf(" %d ", node->data);        last = node;        node = node->next;    }      printf("\nTraversal in reverse direction \n");    while (last != NULL) {        printf(" %d ", last->data);        last = last->prev;    }}  /* Driver program to test above functions*/int main(){    /* Start with the empty list */    struct Node* head = NULL;      // Insert 6.  So linked list becomes 6->NULL    append(&head, 6);      // Insert 7 at the beginning. So linked list becomes    // 7->6->NULL    push(&head, 7);      // Insert 1 at the beginning. So linked list becomes    // 1->7->6->NULL    push(&head, 1);      // Insert 4 at the end. So linked list becomes    // 1->7->6->4->NULL    append(&head, 4);      // Insert 8, after 7. So linked list becomes    // 1->7->8->6->4->NULL    insertAfter(head->next, 8);      printf("Created DLL is: ");    printList(head);      getchar();    return 0;}
// A complete working Java program to demonstrate all  // Class for Doubly Linked Listpublic class DLL {    Node head; // head of list      /* Doubly Linked list Node*/    class Node {        int data;        Node prev;        Node next;          // Constructor to create a new node        // next and prev is by default initialized as null        Node(int d) { data = d; }    }      // Adding a node at the front of the list    public void push(int new_data)    {        /* 1. allocate node         * 2. put in the data */        Node new_Node = new Node(new_data);          /* 3. Make next of new node as head and previous as NULL */        new_Node.next = head;        new_Node.prev = null;          /* 4. change prev of head node to new node */        if (head != null)            head.prev = new_Node;          /* 5. move the head to point to the new node */        head = new_Node;    }        // Add a node before the given node    public void InsertBefore(Node next_node, int new_data)    {        /*Check if the given nx_node is NULL*/        if(next_node == null)        {            System.out.println("The given next node can not be NULL");            return;        }                  //Allocate node, put in the data        Node new_node = new Node(new_data);                  //Making prev of new node as prev of next node        new_node.prev = next_node.prev;                  //Making prev of next node as new node        next_node.prev = new_node;                  //Making next of new node as next node        new_node.next = next_node;                  //Check if new node is added as head        if(new_node.prev != null)            new_node.prev.next = new_node;        else            head = new_node;    }      /* Given a node as prev_node, insert     a new node after the given node */    public void InsertAfter(Node prev_Node, int new_data)    {          /*1. check if the given prev_node is NULL */        if (prev_Node == null) {            System.out.println("The given previous node cannot be NULL ");            return;        }          /* 2. allocate node         * 3. put in the data */        Node new_node = new Node(new_data);          /* 4. Make next of new node as next of prev_node */        new_node.next = prev_Node.next;          /* 5. Make the next of prev_node as new_node */        prev_Node.next = new_node;          /* 6. Make prev_node as previous of new_node */        new_node.prev = prev_Node;          /* 7. Change previous of new_node's next node */        if (new_node.next != null)            new_node.next.prev = new_node;                  /*8. Maintaining tail node if we are inserting after the second last node */          if (newNode.next==null){            tail = newNode;        }    }      // Add a node at the end of the list    void append(int new_data)    {        /* 1. allocate node         * 2. put in the data */        Node new_node = new Node(new_data);          Node last = head; /* used in step 5*/          /* 3. This new node is going to be the last node, so        * make next of it as NULL*/        new_node.next = null;          /* 4. If the Linked List is empty, then make the new        * node as head */        if (head == null) {            new_node.prev = null;            head = new_node;            return;        }          /* 5. Else traverse till the last node */        while (last.next != null)            last = last.next;          /* 6. Change the next of last node */        last.next = new_node;          /* 7. Make last node as previous of new node */        new_node.prev = last;    }      // This function prints contents of     // linked list starting from the given node    public void printlist(Node node)    {        Node last = null;        System.out.println("Traversal in forward Direction");        while (node != null) {            System.out.print(node.data + " ");            last = node;            node = node.next;        }        System.out.println();        System.out.println("Traversal in reverse direction");        while (last != null) {            System.out.print(last.data + " ");            last = last.prev;        }    }      /* Driver program to test above functions*/    public static void main(String[] args)    {        /* Start with the empty list */        DLL dll = new DLL();          // Insert 6. So linked list becomes 6->NULL        dll.append(6);          // Insert 7 at the beginning. So         // linked list becomes 7->6->NULL        dll.push(7);          // Insert 1 at the beginning. So         // linked list becomes 1->7->6->NULL        dll.push(1);          // Insert 4 at the end. So linked         // list becomes 1->7->6->4->NULL        dll.append(4);          // Insert 8, after 7. So linked         // list becomes 1->7->8->6->4->NULL        dll.InsertAfter(dll.head.next, 8);                // Insert 5, before 8.So linked        // list becomes 1->7->5->8->6->4        dll.InsertBefore(dll.head.next.next, 5);          System.out.println("Created DLL is: ");        dll.printlist(dll.head);    }}  // This code is contributed by Sumit Ghosh
# A complete working Python # program to demonstrate all# insertion methods  # A linked list nodeclass Node:      # Constructor to create a new node    def __init__(self, data):        self.data = data        self.next = None        self.prev = None  # Class to create a Doubly Linked Listclass DoublyLinkedList:      # Constructor for empty Doubly Linked List    def __init__(self):        self.head = None      # Given a reference to the head of a list and an    # integer, inserts a new node on the front of list    def push(self, new_data):          # 1. Allocates node        # 2. Put the data in it        new_node = Node(new_data)          # 3. Make next of new node as head and        # previous as None (already None)        new_node.next = self.head          # 4. change prev of head node to new_node        if self.head is not None:            self.head.prev = new_node          # 5. move the head to point to the new node        self.head = new_node      # Given a node as prev_node, insert a new node after    # the given node    def insertAfter(self, prev_node, new_data):          # 1. Check if the given prev_node is None        if prev_node is None:            print("the given previous node cannot be NULL")            return          # 2. allocate new node        # 3. put in the data        new_node = Node(new_data)          # 4. Make net of new node as next of prev node        new_node.next = prev_node.next          # 5. Make prev_node as previous of new_node        prev_node.next = new_node          # 6. Make prev_node ass previous of new_node        new_node.prev = prev_node          # 7. Change previous of new_nodes's next node        if new_node.next:            new_node.next.prev = new_node      # Given a reference to the head of DLL and integer,    # appends a new node at the end    def append(self, new_data):          # 1. Allocates node        # 2. Put in the data        new_node = Node(new_data)          # 3. This new node is going to be the last node,        # so make next of it as None         # (It already is initialized as None)          # 4. If the Linked List is empty, then make the        # new node as head        if self.head is None:            self.head = new_node            return          # 5. Else traverse till the last node        last = self.head        while last.next:            last = last.next          # 6. Change the next of last node        last.next = new_node          # 7. Make last node as previous of new node        new_node.prev = last          return      # This function prints contents of linked list    # starting from the given node    def printList(self, node):          print("\nTraversal in forward direction")        while node:            print(" {}".format(node.data))            last = node            node = node.next          print("\nTraversal in reverse direction")        while last:            print(" {}".format(last.data))            last = last.prev  # Driver program to test above functions    # Start with empty listllist = DoublyLinkedList()  # Insert 6. So the list becomes 6->Nonellist.append(6)  # Insert 7 at the beginning.# So linked list becomes 7->6->Nonellist.push(7)  # Insert 1 at the beginning.# So linked list becomes 1->7->6->Nonellist.push(1)  # Insert 4 at the end.# So linked list becomes 1->7->6->4->Nonellist.append(4)  # Insert 8, after 7.# So linked list becomes 1->7->8->6->4->Nonellist.insertAfter(llist.head.next, 8)  print ("Created DLL is: ")llist.printList(llist.head)  # This code is contributed by Nikhil Kumar Singh(nickzuck_007)
// A complete working C# program to demonstrate allusing System;   // Class for Doubly Linked List public class DLL {     Node head; // head of list       /* Doubly Linked list Node*/    public class Node     {         public int data;         public Node prev;         public Node next;           // Constructor to create a new node         // next and prev is by default initialized as null         public Node(int d)         {             data = d;        }     }       // Adding a node at the front of the list     public void push(int new_data)     {         /* 1. allocate node         * 2. put in the data */        Node new_Node = new Node(new_data);           /* 3. Make next of new node as        head and previous as NULL */        new_Node.next = head;         new_Node.prev = null;           /* 4. change prev of head node to new node */        if (head != null)             head.prev = new_Node;           /* 5. move the head to point to the new node */        head = new_Node;     }       /* Given a node as prev_node, insert    a new node after the given node */    public void InsertAfter(Node prev_Node, int new_data)     {           /*1. check if the given prev_node is NULL */        if (prev_Node == null)        {             Console.WriteLine("The given previous node cannot be NULL ");             return;         }           /* 2. allocate node         * 3. put in the data */        Node new_node = new Node(new_data);           /* 4. Make next of new node as next of prev_node */        new_node.next = prev_Node.next;           /* 5. Make the next of prev_node as new_node */        prev_Node.next = new_node;           /* 6. Make prev_node as previous of new_node */        new_node.prev = prev_Node;           /* 7. Change previous of new_node's next node */        if (new_node.next != null)             new_node.next.prev = new_node;     }       // Add a node at the end of the list     void append(int new_data)     {         /* 1. allocate node         * 2. put in the data */        Node new_node = new Node(new_data);           Node last = head; /* used in step 5*/          /* 3. This new node is going             to be the last node, so         * make next of it as NULL*/        new_node.next = null;           /* 4. If the Linked List is empty,         then make the new * node as head */        if (head == null)         {             new_node.prev = null;             head = new_node;             return;         }           /* 5. Else traverse till the last node */        while (last.next != null)             last = last.next;           /* 6. Change the next of last node */        last.next = new_node;           /* 7. Make last node as previous of new node */        new_node.prev = last;     }       // This function prints contents of     // linked list starting from the given node     public void printlist(Node node)     {         Node last = null;         Console.WriteLine("Traversal in forward Direction");         while (node != null) {             Console.Write(node.data + " ");             last = node;             node = node.next;         }         Console.WriteLine();         Console.WriteLine("Traversal in reverse direction");         while (last != null) {             Console.Write(last.data + " ");             last = last.prev;         }     }       /* Driver code*/    public static void Main(String[] args)     {         /* Start with the empty list */        DLL dll = new DLL();           // Insert 6. So linked list becomes 6->NULL         dll.append(6);           // Insert 7 at the beginning.         // So linked list becomes 7->6->NULL         dll.push(7);           // Insert 1 at the beginning.         // So linked list becomes 1->7->6->NULL         dll.push(1);           // Insert 4 at the end. So linked list        // becomes 1->7->6->4->NULL         dll.append(4);           // Insert 8, after 7. So linked list         // becomes 1->7->8->6->4->NULL         dll.InsertAfter(dll.head.next, 8);           Console.WriteLine("Created DLL is: ");         dll.printlist(dll.head);     } }   // This code is contributed by 29AjayKumar
<script>// A complete working javascript program to demonstrate all  // Class for Doubly Linked List      var head; // head of list      /* Doubly Linked list Node */    class Node {          // Constructor to create a new node        // next and prev is by default initialized as nullconstructor(d) {            this.data = d;            this.next = null;            this.prev = null;        }    }      // Adding a node at the front of the list     function push(new_data) {        /*         * 1. allocate node 2. put in the data         */var new_Node = new Node(new_data);          /* 3. Make next of new node as head and previous as NULL */        new_Node.next = head;        new_Node.prev = null;          /* 4. change prev of head node to new node */        if (head != null)            head.prev = new_Node;          /* 5. move the head to point to the new node */        head = new_Node;    }      // Add a node before the given node     function InsertBefore(next_node , new_data) {        /* Check if the given nx_node is NULL */        if (next_node == null) {            document.write("The given next node can not be NULL");            return;        }          // Allocate node, put in the datavar new_node = new Node(new_data);          // Making prev of new node as prev of next node        new_node.prev = next_node.prev;          // Making prev of next node as new node        next_node.prev = new_node;          // Making next of new node as next node        new_node.next = next_node;          // Check if new node is added as head        if (new_node.prev != null)            new_node.prev.next = new_node;        else            head = new_node;    }      /*     * Given a node as prev_node, insert a new node after the given node     */     function InsertAfter(prev_Node , new_data) {          /* 1. check if the given prev_node is NULL */        if (prev_Node == null) {            document.write("The given previous node cannot be NULL ");            return;        }          /*         * 2. allocate node 3. put in the data         */var new_node = new Node(new_data);          /* 4. Make next of new node as next of prev_node */        new_node.next = prev_Node.next;          /* 5. Make the next of prev_node as new_node */        prev_Node.next = new_node;          /* 6. Make prev_node as previous of new_node */        new_node.prev = prev_Node;          /* 7. Change previous of new_node's next node */        if (new_node.next != null)            new_node.next.prev = new_node;    }      // Add a node at the end of the list    function append(new_data) {        /*         * 1. allocate node 2. put in the data         */var new_node = new Node(new_data);  var last = head; /* used in step 5 */          /*         * 3. This new node is going to be the last node, so make next of it as NULL         */        new_node.next = null;          /*         * 4. If the Linked List is empty, then make the new node as head         */        if (head == null) {            new_node.prev = null;            head = new_node;            return;        }          /* 5. Else traverse till the last node */        while (last.next != null)            last = last.next;          /* 6. Change the next of last node */        last.next = new_node;          /* 7. Make last node as previous of new node */        new_node.prev = last;    }      // This function prints contents of    // linked list starting from the given node     function printlist(node) {var last = null;        document.write("<br/>Traversal in forward Direction<br/>");        while (node != null) {            document.write(node.data + " ");            last = node;            node = node.next;        }        document.write();        document.write("<br/>Traversal in reverse direction<br/>");        while (last != null) {            document.write(last.data + " ");            last = last.prev;        }    }      /* Driver program to test above functions */              /* Start with the empty list */                  // Insert 6. So linked list becomes 6->NULL        append(6);          // Insert 7 at the beginning. So        // linked list becomes 7->6->NULL        push(7);          // Insert 1 at the beginning. So        // linked list becomes 1->7->6->NULL        push(1);          // Insert 4 at the end. So linked        // list becomes 1->7->6->4->NULL        append(4);          // Insert 8, after 7. So linked        // list becomes 1->7->8->6->4->NULL        InsertAfter(head.next, 8);          // Insert 5, before 8.So linked        // list becomes 1->7->5->8->6->4        InsertBefore(head.next.next, 5);          document.write("Created DLL is:<br/> ");        printlist(head);  // This code is contributed by Rajput-Ji</script>
Created DLL is: 
Traversal in forward direction 
 1  7  8  6  4 
Traversal in reverse direction 
 4  6  8  7  1 
However there is another method which uses constructor call inside the node class in order to minimize the memory allocation work. It also minimizes the number of lines of code.
C++
// Divyansh Mishra --> divyanshmishra101010#include <iostream>using namespace std;    class node{    public:    node* prev;    int data;    node* next;                node(int value){  // A constructor is called here        prev=NULL;      // By default previous pointer is pointed to NULL        data=value;   // value is assigned to the data        next=NULL;    // By default next pointer is pointed to NULL    }};    void insert_at_head(node* &head, int value){          node* n = new node(value);    n->next=head;      if(head!=NULL){        head->prev=n;    }          head=n;}    void insert_at_tail(node* &head, int value){          if(head==NULL){        insert_at_head(head, value);        return;    }          node* n = new node(value);    node* temp=head;              while(temp->next!=NULL){        temp=temp->next;    }    temp->next=n;    n->prev=temp;}    void display(node* head){    node* temp=head;    while(temp!=NULL){        cout<<temp->data<<" --> ";        temp=temp->next;    }    cout<<"NULL"<<endl;}    int main(){    node* head=NULL;      // declaring an empty doubly linked list        insert_at_tail(head,1);    insert_at_tail(head,2);    insert_at_tail(head,3);    insert_at_tail(head,4);    insert_at_tail(head,5);            cout<<"After insertion at tail: ";    display(head);            cout<<"After insertion at head: ";    insert_at_head(head,0);          display(head);      return 0;}
After insertion at tail: 1 --> 2 --> 3 --> 4 --> 5 --> NULL
After insertion at head: 0 --> 1 --> 2 --> 3 --> 4 --> 5 --> NULL
Also see: Delete a node in double Link List Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. 
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