Linked List | Set 2 (Inserting a node)

24 Jun, 2022
We have introduced Linked Lists in the previous post. We also created a simple linked list with 3 nodes and discussed linked list traversal.All programs discussed in this post consider the following representations of linked list. 
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// A linked list node class Node {     public:    int data;     Node *next; }; // This code is contributed by rathbhupendra
// A linked list nodestruct Node{  int data;  struct Node *next;};
// Linked List Classclass LinkedList{    Node head;  // head of list      /* Node Class */    class Node    {        int data;        Node next;                   // Constructor to create a new node        Node(int d) {data = d; next = null; }    }}
# Node classclass Node:      # Function to initialize the node object    def __init__(self, data):        self.data = data  # Assign data        self.next = None  # Initialize next as null  # Linked List classclass LinkedList:        # Function to initialize the Linked List object    def __init__(self):         self.head = None
/* Linked list Node*/public class Node{    public int data;    public Node next;    public Node(int d) {data = d; next = null; }}
<script>// Linked List Class    var head; // head of list      /* Node Class */    class Node {          // Constructor to create a new node        constructor(d) {            this.data = d;            this.next = null;        }    }// This code is contributed by todaysgaurav </script>
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In this post, methods to insert a new node in linked list are discussed. A node can be added in three ways 1) At the front of the linked list  2) After a given node. 3) At the end of the linked list.
Add a node at the front: (4 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 the Linked List. For example, if the given Linked List is 10->15->20->25 and we add an item 5 at the front, then the Linked List becomes 5->10->15->20->25. 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 4 steps to add a 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 */    new_node->next = (*head_ref);       /* 4. move the head to point to the new node */    (*head_ref) = new_node; }   // This code is contributed by rathbhupendra
/* 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 */    new_node->next = (*head_ref);       /* 4. move the head to point to the new node */    (*head_ref)    = new_node;}
/* This function is in LinkedList class. Inserts a   new Node at front of the list. This method is    defined inside LinkedList class shown above */public void push(int new_data){    /* 1 & 2: Allocate the Node &              Put in the data*/    Node new_node = new Node(new_data);      /* 3. Make next of new Node as head */    new_node.next = head;      /* 4. Move the head to point to new Node */    head = new_node;}
# This function is in LinkedList class# Function to insert a new node at the beginningdef push(self, new_data):      # 1 & 2: Allocate the Node &    #        Put in the data    new_node = Node(new_data)              # 3. Make next of new Node as head    new_node.next = self.head              # 4. Move the head to point to new Node     self.head = new_node
/* Inserts a new Node at front of the list. */public void push(int new_data){    /* 1 & 2: Allocate the Node &               Put in the data*/    Node new_node = new Node(new_data);      /* 3. Make next of new Node as head */    new_node.next = head;      /* 4. Move the head to point to new Node */    head = new_node;}
<script>  /* This function is in LinkedList class. Inserts a   new Node at front of the list. This method is    defined inside LinkedList class shown above */      function push(new_data){    /* 1 & 2: Allocate the Node &              Put in the data*/    var new_node = new Node(new_data);      /* 3. Make next of new Node as head */    new_node.next = head;      /* 4. Move the head to point to new Node */    head = new_node;}  // This code contributed by Rajput-Ji   </script>
We have a pointer to the head and we can directly attach a node and change the pointer. So the Time complexity of inserting a node at head position is O(1) as it does a constant amount of work.
Add a node after a given node: (5 steps process) We are given a pointer to a node, and the new node is inserted after the given node. 
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// Given a node prev_node, insert a// new node after the given// prev_nodevoid 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. move the next of prev_node    // as new_node    prev_node->next = new_node;}  // This code is contributed by anmolgautam818,// arkajyotibasak
/* Given a node prev_node, insert a new node after the givenprev_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. move the next of prev_node as new_node */    prev_node->next = new_node;}
/* This function is in LinkedList class.Inserts a new node after the given prev_node. This method isdefined inside LinkedList class shown above */public void insertAfter(Node prev_node, int new_data){    /* 1. Check if the given Node is null */    if (prev_node == null) {        System.out.println(            "The given previous node cannot be null");        return;    }      /* 2. Allocate the 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 next of prev_node as new_node */    prev_node.next = new_node;}
# This function is in LinkedList class.# Inserts a new node after the given prev_node. This method is# defined inside LinkedList class shown above */    def insertAfter(self, prev_node, new_data):      # 1. check if the given prev_node exists    if prev_node is None:        print("The given previous node must inLinkedList.")        return      # 2. Create new node &    # 3. Put in the data    new_node = Node(new_data)      # 4. Make next of new Node as next of prev_node    new_node.next = prev_node.next      # 5. make next of prev_node as new_node    prev_node.next = new_node
/* Inserts a new node after the given prev_node. */public void insertAfter(Node prev_node, int new_data){    /* 1. Check if the given Node is null */    if (prev_node == null) {        Console.WriteLine("The given previous node"                          + " cannot be null");        return;    }      /* 2 & 3: Allocate the Node &            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 next of prev_node                    as new_node */    prev_node.next = new_node;}
<script>  /* This function is in LinkedList class. Inserts a new node after the given prev_node. This method is defined inside LinkedList class shown above */function insertAfter(prev_node , new_data) {       /* 1. Check if the given Node is null */    if (prev_node == null)     {         document.write("The given previous node cannot be null");         return;     }       /* 2. Allocate the 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 next of prev_node as new_node */    prev_node.next = new_node; }  // This code is contributed by aashish1995  </script>
Time complexity of insertAfter() is O(n) as it depends on n where n is the size of the linked list
Space complexity: O(1) since using constant space to modify pointers
Add a node at the end: (6 steps process) The new node is always added after the last node of the given Linked List. For example if the given Linked List is 5->10->15->20->25 and we add an item 30 at the end, then the Linked List becomes 5->10->15->20->25->30. Since a Linked List is typically represented by the head of it, we have to traverse the list till the end and then change the next to last node to a new node. 
Following are the 6 steps to add node at the end.
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// Given a reference (pointer to pointer) to the head  // of a list 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();         // Used in step 5     Node *last = *head_ref;         // 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)      {          *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;      return;  }    // This code is contributed by anmolgautam818, arkajyotibasak
/* Given a reference (pointer to pointer) to the head   of a list 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)    {       *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;    return;    }
/* Appends a new node at the end.  This method is    defined inside LinkedList class shown above */public void append(int new_data){    /* 1. Allocate the Node &       2. Put in the data       3. Set next as null */    Node new_node = new Node(new_data);      /* 4. If the Linked List is empty, then make the           new node as head */    if (head == null)    {        head = new Node(new_data);        return;    }      /* 4. This new node is going to be the last node, so         make next of it as null */    new_node.next = null;      /* 5. Else traverse till the last node */    Node last = head;     while (last.next != null)        last = last.next;      /* 6. Change the next of last node */    last.next = new_node;    return;}
# This function is defined in Linked List class# Appends a new node at the end.  This method is# defined inside LinkedList class shown above    def append(self, new_data):          # 1. Create a new node        # 2. Put in the data        # 3. Set next as None        new_node = Node(new_data)          # 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
/* Appends a new node at the end. This method is defined inside LinkedList class shown above */public void append(int new_data){    /* 1. Allocate the Node &    2. Put in the data    3. Set next as null */    Node new_node = new Node(new_data);      /* 4. If the Linked List is empty,        then make the new node as head */    if (head == null)    {        head = new Node(new_data);        return;    }      /* 4. This new node is going to be     the last node, so make next of it as null */    new_node.next = null;      /* 5. Else traverse till the last node */    Node last = head;     while (last.next != null)        last = last.next;      /* 6. Change the next of last node */    last.next = new_node;    return;}
<script>  /* Appends a new node at the end.  This method is    defined inside LinkedList class shown above */ function append(new_data){    /* 1. Allocate the Node &       2. Put in the data       3. Set next as null */    var new_node = new Node(new_data);      /* 4. If the Linked List is empty, then make the           new node as head */    if (head == null)    {        head = new Node(new_data);        return;    }      /* 4. This new node is going to be the last node, so         make next of it as null */    new_node.next = null;      /* 5. Else traverse till the last node */    var last = head;     while (last.next != null)        last = last.next;      /* 6. Change the next of last node */    last.next = new_node;    return;}  // This code contributed by aashish1995</script>
Time complexity of append is O(n) where n is the number of nodes in the linked list. Since there is a loop from head to end, the function does O(n) work. This method can also be optimized to work in O(1) by keeping an extra pointer to the tail of the linked list/
Following is a complete program that uses all of the above methods to create a linked list.
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// A complete working C++ program to demonstrate //  all insertion methods on Linked List #include <bits/stdc++.h>using namespace std;  // A linked list node class Node {     public:    int data;     Node *next; };   /* Given a reference (pointer to pointer)to the head of a list and an int, insertsa 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 */    new_node->next = (*head_ref);       /* 4. move the head to point to the new node */    (*head_ref) = new_node; }   /* Given a node prev_node, insert a new node after the given prev_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. move the next of prev_node as new_node */    prev_node->next = new_node; }   /* Given a reference (pointer to pointer) to the head of a list 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)     {         *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;     return; }   // This function prints contents of// linked list starting from head void printList(Node *node) {     while (node != NULL)     {         cout<<" "<<node->data;         node = node->next;     } }   /* Driver code*/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 Linked list is: ";     printList(head);           return 0; }     // This code is contributed by rathbhupendra, arkajyotibasak
// A complete working C program to demonstrate all insertion methods// on Linked List#include <stdio.h>#include <stdlib.h>  // A linked list nodestruct Node{  int data;  struct Node *next;};  /* 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 */    new_node->next = (*head_ref);      /* 4. move the head to point to the new node */    (*head_ref)    = new_node;}  /* Given a node prev_node, insert a new node after the given    prev_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. move the next of prev_node as new_node */    prev_node->next = new_node;}  /* Given a reference (pointer to pointer) to the head   of a list 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)    {       *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;    return;}  // This function prints contents of linked list starting from headvoid printList(struct Node *node){  while (node != NULL)  {     printf(" %d ", node->data);     node = node->next;  }}  /* 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("\n Created Linked list is: ");  printList(head);    return 0;}
// A complete working Java program to demonstrate all insertion methods// on linked listclass LinkedList{    Node head;  // head of list      /* Linked list Node*/    class Node    {        int data;        Node next;        Node(int d) {data = d; next = null; }    }      /* Inserts a new Node at front of the list. */    public void push(int new_data)    {        /* 1 & 2: Allocate the Node &                  Put in the data*/        Node new_node = new Node(new_data);          /* 3. Make next of new Node as head */        new_node.next = head;          /* 4. Move the head to point to new Node */        head = new_node;    }      /* Inserts a new node after the given prev_node. */    public void insertAfter(Node prev_node, int new_data)    {        /* 1. Check if the given Node is null */        if (prev_node == null)        {            System.out.println("The given previous node cannot be null");            return;        }          /* 2 & 3: Allocate the Node &                  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 next of prev_node as new_node */        prev_node.next = new_node;    }         /* Appends a new node at the end.  This method is        defined inside LinkedList class shown above */    public void append(int new_data)    {        /* 1. Allocate the Node &           2. Put in the data           3. Set next as null */        Node new_node = new Node(new_data);          /* 4. If the Linked List is empty, then make the              new node as head */        if (head == null)        {            head = new Node(new_data);            return;        }          /* 4. This new node is going to be the last node, so              make next of it as null */        new_node.next = null;          /* 5. Else traverse till the last node */        Node last = head;         while (last.next != null)            last = last.next;          /* 6. Change the next of last node */        last.next = new_node;        return;    }      /* This function prints contents of linked list starting from        the given node */    public void printList()    {        Node tnode = head;        while (tnode != null)        {            System.out.print(tnode.data+" ");            tnode = tnode.next;        }    }      /* Driver program to test above functions. Ideally this function       should be in a separate user class.  It is kept here to keep       code compact */    public static void main(String[] args)    {        /* Start with the empty list */        LinkedList llist = new LinkedList();          // Insert 6.  So linked list becomes 6->NUllist        llist.append(6);          // Insert 7 at the beginning. So linked list becomes        // 7->6->NUllist        llist.push(7);          // Insert 1 at the beginning. So linked list becomes        // 1->7->6->NUllist        llist.push(1);          // Insert 4 at the end. So linked list becomes        // 1->7->6->4->NUllist        llist.append(4);          // Insert 8, after 7. So linked list becomes        // 1->7->8->6->4->NUllist        llist.insertAfter(llist.head.next, 8);          System.out.println("\nCreated Linked list is: ");        llist.printList();    }}// This code is contributed by Rajat Mishra
# A complete working Python program to demonstrate all# insertion methods of linked list  # Node classclass Node:      # Function to initialise the node object    def __init__(self, data):        self.data = data  # Assign data        self.next = None  # Initialize next as null    # Linked List class contains a Node objectclass LinkedList:      # Function to initialize head    def __init__(self):        self.head = None        # Function to insert a new node at the beginning    def push(self, new_data):          # 1 & 2: Allocate the Node &        #        Put in the data        new_node = Node(new_data)          # 3. Make next of new Node as head        new_node.next = self.head          # 4. Move the head to point to new Node        self.head = new_node        # This function is in LinkedList class. Inserts a    # new node after the given prev_node. This method is    # defined inside LinkedList class shown above */    def insertAfter(self, prev_node, new_data):          # 1. check if the given prev_node exists        if prev_node is None:            print("The given previous node must inLinkedList.")            return          #  2. create new node &        #      Put in the data        new_node = Node(new_data)          # 4. Make next of new Node as next of prev_node        new_node.next = prev_node.next          # 5. make next of prev_node as new_node        prev_node.next = new_node        # This function is defined in Linked List class    # Appends a new node at the end.  This method is    # defined inside LinkedList class shown above */    def append(self, new_data):          # 1. Create a new node        # 2. Put in the data        # 3. Set next as None        new_node = Node(new_data)          # 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        # Utility function to print the linked list    def printList(self):        temp = self.head        while (temp):            print(temp.data,end=" ")            temp = temp.next      # Code execution starts hereif __name__=='__main__':      # Start with the empty list    llist = LinkedList()      # Insert 6.  So linked list becomes 6->None    llist.append(6)      # Insert 7 at the beginning. So linked list becomes 7->6->None    llist.push(7);      # Insert 1 at the beginning. So linked list becomes 1->7->6->None    llist.push(1);      # Insert 4 at the end. So linked list becomes 1->7->6->4->None    llist.append(4)      # Insert 8, after 7. So linked list becomes 1 -> 7-> 8-> 6-> 4-> None    llist.insertAfter(llist.head.next, 8)      print('Created linked list is: ')    llist.printList()  # This code is contributed by Manikantan Narasimhan
// A complete working C# program to demonstrate // all insertion methods on linked listusing System;      class GFG{    public Node head; // head of list      /* Linked list Node*/    public class Node    {        public int data;        public Node next;        public Node(int d) {data = d; next = null;}    }      /* Inserts a new Node at front of the list. */    public void push(int new_data)    {        /* 1 & 2: Allocate the Node &                Put in the data*/        Node new_node = new Node(new_data);          /* 3. Make next of new Node as head */        new_node.next = head;          /* 4. Move the head to point to new Node */        head = new_node;    }      /* Inserts a new node after the given prev_node. */    public void insertAfter(Node prev_node, int new_data)    {        /* 1. Check if the given Node is null */        if (prev_node == null)        {            Console.WriteLine("The given previous" +                               " node cannot be null");            return;        }          /* 2 & 3: Allocate the Node &                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 next of prev_node as new_node */        prev_node.next = new_node;    }          /* Appends a new node at the end. This method is     defined inside LinkedList class shown above */    public void append(int new_data)    {        /* 1. Allocate the Node &        2. Put in the data        3. Set next as null */        Node new_node = new Node(new_data);          /* 4. If the Linked List is empty,        then make the new node as head */        if (head == null)        {            head = new Node(new_data);            return;        }          /* 4. This new node is going to be the last node,             so make next of it as null */        new_node.next = null;          /* 5. Else traverse till the last node */        Node last = head;         while (last.next != null)            last = last.next;          /* 6. Change the next of last node */        last.next = new_node;        return;    }      /* This function prints contents of linked list     starting from the given node */    public void printList()    {        Node tnode = head;        while (tnode != null)        {            Console.Write(tnode.data + " ");            tnode = tnode.next;        }    }      // Driver Code    public static void Main(String[] args)    {        /* Start with the empty list */        GFG llist = new GFG();          // Insert 6. So linked list becomes 6->NUllist        llist.append(6);          // Insert 7 at the beginning.         // So linked list becomes 7->6->NUllist        llist.push(7);          // Insert 1 at the beginning.         // So linked list becomes 1->7->6->NUllist        llist.push(1);          // Insert 4 at the end. So linked list becomes        // 1->7->6->4->NUllist        llist.append(4);          // Insert 8, after 7. So linked list becomes        // 1->7->8->6->4->NUllist        llist.insertAfter(llist.head.next, 8);          Console.Write("Created Linked list is: ");        llist.printList();    }}  // This code is contributed by Rajput-Ji
<script>  // A complete working javascript program // to demonstrate all insertion methods// on linked list     var head; // head of list      /* Linked list Node */     class Node {            constructor(val) {                this.data = val;                this.next = null;            }        }      /* Inserts a new Node at front of the list. */     function push(new_data) {        /*         * 1 & 2: Allocate the Node & Put in the data         */        var new_node = new Node(new_data);          /* 3. Make next of new Node as head */        new_node.next = head;          /* 4. Move the head to point to new Node */        head = new_node;    }      /* Inserts a new node after the given prev_node. */     function insertAfter(prev_node , new_data) {        /* 1. Check if the given Node is null */        if (prev_node == null) {            document.write(            "The given previous node cannot be null"            );            return;        }          /*         * 2 & 3: Allocate the Node & 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 next of prev_node as new_node */        prev_node.next = new_node;    }      /*     * Appends a new node at the end.        This method is defined inside LinkedList class     * shown above     */     function append(new_data) {        /*         * 1. Allocate the Node & 2.          Put in the data 3. Set next as null         */         var new_node = new Node(new_data);          /*         * 4. If the Linked List is empty,          then make the new node as head         */        if (head == null) {            head = new Node(new_data);            return;        }          /*         * 4. This new node is going to be the last node,          so make next of it as null         */        new_node.next = null;          /* 5. Else traverse till the last node */        var last = head;        while (last.next != null)            last = last.next;          /* 6. Change the next of last node */        last.next = new_node;        return;    }      /*     * This function prints contents of linked list     starting from the given node     */     function printList() {var tnode = head;        while (tnode != null) {            document.write(tnode.data + " ");            tnode = tnode.next;        }    }      /*     * Driver program to test above functions.        Ideally this function should be in a     * separate user class. It is kept here     to keep code compact     */              /* Start with the empty list */          // Insert 6. So linked list becomes 6->NUllist        append(6);          // Insert 7 at the beginning. So linked list becomes        // 7->6->NUllist        push(7);          // Insert 1 at the beginning. So linked list becomes        // 1->7->6->NUllist        push(1);          // Insert 4 at the end. So linked list becomes        // 1->7->6->4->NUllist        append(4);          // Insert 8, after 7. So linked list becomes        // 1->7->8->6->4->NUllist        insertAfter(head.next, 8);          document.write("\nCreated Linked list is: ");        printList();  // This code contributed by gauravrajput1  </script>
Output:
 Created Linked list is:  1  7  8  6  4
Time Complexity : O(n) Auxiliary Space : O(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++
// Alternate method to declare the class in order to minimize the memory allocation work  #include <iostream>using namespace std;  class node {public:    int data;    node* next;      node(int value)        // A constructor is called here    {                             data = value;     // it automatic assigns the value to  the data        next = NULL;     // next pointer is pointed to NULL    }};          // function to insert a element at head positionvoid insertathead(node*& head, int val){    node* n = new node(val);    n->next = head;    head = n;}          // function to insert a element at a specified positionvoid insertafter(node* head, int key, int val){    node* n = new node(val);    if (key == head->data) {        n->next = head->next;        head->next = n;        return;    }      node* temp = head;    while (temp->data != key) {        temp = temp->next;        if (temp == NULL) {            return;        }    }    n->next = temp->next;    temp->next = n;}          // function to insert a element at the endvoid insertattail(node*& head, int val){    node* n = new node(val);    if (head == NULL) {        head = n;        return;    }      node* temp = head;    while (temp->next != NULL) {        temp = temp->next;    }    temp->next = n;}          // function to print the singly linked listvoid print(node*& head){    node* temp = head;      while (temp != NULL) {        cout << temp->data << " -> ";        temp = temp->next;    }    cout << "NULL" << endl;}        // main functionint main(){    node* head = NULL;     //declaring an empty linked list      insertathead(head, 1);    insertathead(head, 2);    cout << "After insertion at head: ";    print(head);    cout << endl;      insertattail(head, 4);    insertattail(head, 5);    cout << "After insertion at tail: ";    print(head);    cout << endl;      insertafter(head, 1, 2);    insertafter(head, 5, 6);    cout << "After insertion at a given position: ";    print(head);    cout << endl;      return 0;}  // contributed by divyanshmishra101010
After insertion at head: 2 -> 1 -> NULL

After insertion at tail: 2 -> 1 -> 4 -> 5 -> NULL

After insertion at a given position: 2 -> 1 -> 2 -> 4 -> 5 -> 6 -> NULL
Time Complexity : O(n) Auxiliary Space : O(1) 
You may like to try Practice MCQ Questions on Linked 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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