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What is Linked List

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  • Difficulty Level : Easy
  • Last Updated : 14 Sep, 2022
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Like arrays, Linked List is a linear data structure. Unlike arrays, linked list elements are not stored at a contiguous location; the elements are linked using pointers. They include a series of connected nodes. Here, each node stores the data and the address of the next node.

Linked-List

Why Linked List? 

Arrays can be used to store linear data of similar types, but arrays have the following limitations:

  • The size of the arrays is fixed: So we must know the upper limit on the number of elements in advance. Also, generally, the allocated memory is equal to the upper limit irrespective of the usage. 
  • Insertion of a new element / Deletion of a existing element in an array of elements is expensive: The room has to be created for the new elements and to create room existing elements have to be shifted but in Linked list if we have the head node then we can traverse to any node through it and insert new node at the required position.

Example: 
In a system, if we maintain a sorted list of IDs in an array id[] = [1000, 1010, 1050, 2000, 2040]. 
If we want to insert a new ID 1005, then to maintain the sorted order, we have to move all the elements after 1000 (excluding 1000). 

Deletion is also expensive with arrays until unless some special techniques are used. For example, to delete 1010 in id[], everything after 1010 has to be moved due to this so much work is being done which affects the efficiency of the code.

 

Advantages of Linked Lists over arrays:

  • Dynamic Array.
  • Ease of Insertion/Deletion.

Drawbacks of Linked Lists: 

  • Random access is not allowed. We have to access elements sequentially starting from the first node(head node). So we cannot do a binary search with linked lists efficiently with its default implementation. 
  • Extra memory space for a pointer is required with each element of the list. 
  • Not cache friendly. Since array elements are contiguous locations, there is locality of reference which is not there in case of linked lists.

Types of Linked Lists:

  • Simple Linked List – In this type of linked list, one can move or traverse the linked list in only one direction
  • Doubly Linked List – In this type of linked list, one can move or traverse the linked list in both directions (Forward and Backward)
  • Circular Linked List – In this type of linked list, the last node of the linked list contains the link of the first/head node of the linked list in its next pointer and the first/head node contains the link of the last node of the linked list in its prev pointer

Basic operations on Linked Lists:

Representation of Linked Lists: 

A linked list is represented by a pointer to the first node of the linked list. The first node is called the head of the linked list. If the linked list is empty, then the value of the head points to NULL. 

Each node in a list consists of at least two parts: 

  • A Data Item (we can store integer, strings, or any type of data).
  • Pointer (Or Reference) to the next node (connects one node to another) or An address of another node

In C, we can represent a node using structures. Below is an example of a linked list node with integer data. 
In Java or C#, LinkedList can be represented as a class and a Node as a separate class. The LinkedList class contains a reference of Node class type. 

C




// A linked list node
struct Node {
    int data;
    struct Node* next;
};


C++




class Node {
public:
    int data;
    Node* next;
};


Java




class LinkedList {
    Node head; // head of the list
  
    /* Linked list Node*/
    class Node {
        int data;
        Node next;
  
        // Constructor to create a new node
        // Next is by default initialized
        // as null
        Node(int d)
        {
            data = d;
            next = null;
        }
    }
}


Python




# Node class
class 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 class
  
  
class LinkedList:
  
    # Function to initialize the Linked
    # List object
    def __init__(self):
        self.head = None


C#




class LinkedList {
    // The first node(head) of the linked list
    // Will be an object of type Node (null by default)
    Node head;
  
    class Node {
        int data;
        Node next;
  
        // Constructor to create a new node
        Node(int d) { data = d; }
    }
}


Javascript




<script>
var head; // head of the list
  
/* Linked list Node*/
    class Node 
    {
      
         // Constructor to create a new node
        // Next is by default initialized
        // as null
        constructor(val) {
            this.data = val;
            this.next = null;
        }
    }
  
// This code is contributed by gauravrajput1
</script>


Construction of a simple linked list with 3 nodes:

C




// C program to implement a
// linked list
#include <stdio.h>
#include <stdlib.h>
  
struct Node {
    int data;
    struct Node* next;
};
  
// Driver's code
int main()
{
    struct Node* head = NULL;
    struct Node* second = NULL;
    struct Node* third = NULL;
  
    // allocate 3 nodes in the heap
    head = (struct Node*)malloc(sizeof(struct Node));
    second = (struct Node*)malloc(sizeof(struct Node));
    third = (struct Node*)malloc(sizeof(struct Node));
  
    /* Three blocks have been allocated dynamically.
     We have pointers to these three blocks as head,
     second and third
       head           second           third
        |                |               |
        |                |               |
    +---+-----+     +----+----+     +----+----+
    | #  | #  |     | #  | #  |     |  # |  # |
    +---+-----+     +----+----+     +----+----+
  
   # represents any random value.
   Data is random because we haven’t assigned
   anything yet  */
  
    head->data = 1; // assign data in first node
    head->next = second; // Link first node with
    // the second node
  
    /* data has been assigned to the data part of the first
     block (block pointed by the head). And next
     pointer of first block points to second.
     So they both are linked.
  
       head          second         third
        |              |              |
        |              |              |
    +---+---+     +----+----+     +-----+----+
    | 1  | o----->| #  | #  |     |  #  | #  |
    +---+---+     +----+----+     +-----+----+
  */
  
    // assign data to second node
    second->data = 2;
  
    // Link second node with the third node
    second->next = third;
  
    /* data has been assigned to the data part of the second
     block (block pointed by second). And next
     pointer of the second block points to the third
     block. So all three blocks are linked.
  
       head         second         third
        |             |             |
        |             |             |
    +---+---+     +---+---+     +----+----+
    | 1  | o----->| 2 | o-----> |  # |  # |
    +---+---+     +---+---+     +----+----+      */
  
    third->data = 3; // assign data to third node
    third->next = NULL;
  
    /* data has been assigned to data part of third
    block (block pointed by third). And next pointer
    of the third block is made NULL to indicate
    that the linked list is terminated here.
  
     We have the linked list ready.
  
           head
             |
             |
        +---+---+     +---+---+       +----+------+
        | 1  | o----->|  2  | o-----> |  3 | NULL |
        +---+---+     +---+---+       +----+------+
  
  
    Note that only head is sufficient to represent
    the whole list.  We can traverse the complete
    list by following next pointers.    */
  
    return 0;
}


C++




// C++ program to implement a
// linked list
#include <bits/stdc++.h>
using namespace std;
  
class Node {
public:
    int data;
    Node* next;
};
  
// Driver's code
int main()
{
    Node* head = NULL;
    Node* second = NULL;
    Node* third = NULL;
  
    // allocate 3 nodes in the heap
    head = new Node();
    second = new Node();
    third = new Node();
  
    /* Three blocks have been allocated dynamically.
    We have pointers to these three blocks as head,
    second and third
    head         second         third
        |             |             |
        |             |             |
    +---+-----+     +----+----+     +----+----+
    | # | # |     | # | # |     | # | # |
    +---+-----+     +----+----+     +----+----+
  
# represents any random value.
Data is random because we haven’t assigned
anything yet */
  
    head->data = 1; // assign data in first node
    head->next = second; // Link first node with
    // the second node
  
    /* data has been assigned to the data part of first
    block (block pointed by the head). And next
    pointer of the first block points to second.
    So they both are linked.
  
    head         second         third
        |             |             |
        |             |             |
    +---+---+     +----+----+     +-----+----+
    | 1 | o----->| # | # |     | # | # |
    +---+---+     +----+----+     +-----+----+
*/
  
    // assign data to second node
    second->data = 2;
  
    // Link second node with the third node
    second->next = third;
  
    /* data has been assigned to the data part of the second
    block (block pointed by second). And next
    pointer of the second block points to the third
    block. So all three blocks are linked.
  
    head         second         third
        |             |             |
        |             |             |
    +---+---+     +---+---+     +----+----+
    | 1 | o----->| 2 | o-----> | # | # |
    +---+---+     +---+---+     +----+----+     */
  
    third->data = 3; // assign data to third node
    third->next = NULL;
  
    /* data has been assigned to the data part of the third
    block (block pointed by third). And next pointer
    of the third block is made NULL to indicate
    that the linked list is terminated here.
  
    We have the linked list ready.
  
        head
            |
            |
        +---+---+     +---+---+     +----+------+
        | 1 | o----->| 2 | o-----> | 3 | NULL |
        +---+---+     +---+---+     +----+------+
  
  
    Note that only the head is sufficient to represent
    the whole list. We can traverse the complete
    list by following the next pointers. */
  
    return 0;
}
  
// This code is contributed by rathbhupendra


Java




// A simple Java program to introduce a linked list
class LinkedList {
    Node head; // head of list
  
    /* Linked list Node.  This inner class is made static so
       that main() can access it */
    static class Node {
        int data;
        Node next;
        Node(int d)
        {
            data = d;
            next = null;
        } // Constructor
    }
  
    /* method to create a simple linked list with 3 nodes*/
    public static void main(String[] args)
    {
        /* Start with the empty list. */
        LinkedList llist = new LinkedList();
  
        llist.head = new Node(1);
        Node second = new Node(2);
        Node third = new Node(3);
  
        /* Three nodes have been allocated dynamically.
          We have references to these three blocks as head,
          second and third
  
          llist.head        second              third
             |                |                  |
             |                |                  |
         +----+------+     +----+------+     +----+------+
         | 1  | null |     | 2  | null |     |  3 | null |
         +----+------+     +----+------+     +----+------+
       */
  
        llist.head.next = second; // Link first node with
                                  // the second node
  
        /*  Now next of the first Node refers to the second.
        So they both are linked.
  
         llist.head        second              third
            |                |                  |
            |                |                  |
        +----+------+     +----+------+     +----+------+
        | 1  |  o-------->| 2  | null |     |  3 | null |
        +----+------+     +----+------+     +----+------+ */
  
        second.next
            = third; // Link second node with the third node
  
        /*  Now next of the second Node refers to third.  So
        all three nodes are linked.
  
         llist.head        second              third
            |                |                  |
            |                |                  |
        +----+------+     +----+------+     +----+------+
        | 1  |  o-------->| 2  |  o-------->|  3 | null |
        +----+------+     +----+------+     +----+------+ */
    }
}


Python




# A simple Python program to introduce a linked list
  
# Node class
  
  
class 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 object
class LinkedList:
  
    # Function to initialize head
    def __init__(self):
        self.head = None
  
  
# Code execution starts here
if __name__ == '__main__':
  
    # Start with the empty list
    llist = LinkedList()
  
    llist.head = Node(1)
    second = Node(2)
    third = Node(3)
  
    '''
    Three nodes have been created.
    We have references to these three blocks as head,
    second and third
  
    llist.head        second              third
         |                |                  |
         |                |                  |
    +----+------+     +----+------+     +----+------+
    | 1  | None |     | 2  | None |     |  3 | None |
    +----+------+     +----+------+     +----+------+
    '''
  
    llist.head.next = second  # Link first node with second
  
    '''
    Now next of first Node refers to second.  So they
    both are linked.
  
    llist.head        second              third
         |                |                  |
         |                |                  |
    +----+------+     +----+------+     +----+------+
    | 1  |  o-------->| 2  | null |     |  3 | null |
    +----+------+     +----+------+     +----+------+ 
    '''
  
    second.next = third  # Link second node with the third node
  
    '''
    Now next of second Node refers to third.  So all three
    nodes are linked.
  
    llist.head        second              third
         |                |                  |
         |                |                  |
    +----+------+     +----+------+     +----+------+
    | 1  |  o-------->| 2  |  o-------->|  3 | null |
    +----+------+     +----+------+     +----+------+ 
    '''


C#




// A simple C# program to introduce a linked list
using System;
  
public class LinkedList {
    Node head; // head of list
  
    /* Linked list Node. This inner class is made static so
    that main() can access it */
    public class Node {
        public int data;
        public Node next;
        public Node(int d)
        {
            data = d;
            next = null;
        } // Constructor
    }
  
    /* method to create a simple linked list with 3 nodes*/
    public static void Main(String[] args)
    {
        /* Start with the empty list. */
        LinkedList llist = new LinkedList();
  
        llist.head = new Node(1);
        Node second = new Node(2);
        Node third = new Node(3);
  
        /* Three nodes have been allocated dynamically.
        We have references to these three blocks as head,
        second and third
  
        llist.head     second             third
            |             |                 |
            |             |                 |
        +----+------+     +----+------+     +----+------+
        | 1 | null |     | 2 | null |     | 3 | null |
        +----+------+     +----+------+     +----+------+ */
  
        llist.head.next = second; // Link first node with
                                  // the second node
  
        /* Now next of first Node refers to second. So they
            both are linked.
  
        llist.head     second             third
            |             |                 |
            |             |                 |
        +----+------+     +----+------+     +----+------+
        | 1 | o-------->| 2 | null |     | 3 | null |
        +----+------+     +----+------+     +----+------+ */
  
        second.next
            = third; // Link second node with the third node
  
        /* Now next of the second Node refers to third. So
        all three nodes are linked.
  
        llist.head     second             third
            |             |                 |
            |             |                 |
        +----+------+     +----+------+     +----+------+
        | 1 | o-------->| 2 | o-------->| 3 | null |
        +----+------+     +----+------+     +----+------+ */
    }
}
  
// This code has been contributed by 29AjayKumar


Javascript




// A simple javascript program to introduce a linked list
  
    var head; // head of list
  
    /* Linked list Node.  This inner class is made  so that
       main() can access it */
     class Node {
        constructor(d)
        {
            this.data = d;
            this.next = null;
        } // Constructor
    }
  
    /* method to create a simple linked list with 3 nodes*/
      
  
        var head = new Node(1);
        var second = new Node(2);
        var third = new Node(3);
  
        /* Three nodes have been allocated dynamically.
          We have references to these three blocks as head,  
          second and third
  
          llist.head        second              third
             |                |                  |
             |                |                  |
         +----+------+     +----+------+     +----+------+
         | 1  | null |     | 2  | null |     |  3 | null |
         +----+------+     +----+------+     +----+------+ */
  
        head.next = second; // Link first node with the second node
  
        /*  Now next of the first Node refers to the second.  So they
            both are linked.
  
         llist.head        second              third
            |                |                  |
            |                |                  |
        +----+------+     +----+------+     +----+------+
        | 1  |  o-------->| 2  | null |     |  3 | null |
        +----+------+     +----+------+     +----+------+ */
  
        second.next = third; // Link second node with the third node
  
        /*  Now next of the second Node refers to third.  So all three
            nodes are linked.
  
         llist.head        second              third
            |                |                  |
            |                |                  |
        +----+------+     +----+------+     +----+------+
        | 1  |  o-------->| 2  |  o-------->|  3 | null |
        +----+------+     +----+------+     +----+------+ */
   
// This code is contributed by umadevi9616


Traversal of a Linked List

In the previous program, we created a simple linked list with three nodes. Let us traverse the created list and print the data of each node. For traversal, let us write a general-purpose function printList() that prints any given list.

We strongly recommend that you click here and practice it, before moving on to the solution.

C




// A simple C program for
// traversal of a linked list
  
#include <stdio.h>
#include <stdlib.h>
  
struct Node {
    int data;
    struct Node* next;
};
  
// This function prints contents of linked list starting
// from the given node
void printList(struct Node* n)
{
    while (n != NULL) {
        printf(" %d ", n->data);
        n = n->next;
    }
}
  
// Driver's code
int main()
{
    struct Node* head = NULL;
    struct Node* second = NULL;
    struct Node* third = NULL;
  
    // allocate 3 nodes in the heap
    head = (struct Node*)malloc(sizeof(struct Node));
    second = (struct Node*)malloc(sizeof(struct Node));
    third = (struct Node*)malloc(sizeof(struct Node));
  
    head->data = 1; // assign data in first node
    head->next = second; // Link first node with second
  
    second->data = 2; // assign data to second node
    second->next = third;
  
    third->data = 3; // assign data to third node
    third->next = NULL;
  
    // Function call
    printList(head);
  
    return 0;
}


C++




// A simple C++ program for
// traversal of a linked list
  
#include <bits/stdc++.h>
using namespace std;
  
class Node {
public:
    int data;
    Node* next;
};
  
// This function prints contents of linked list
// starting from the given node
void printList(Node* n)
{
    while (n != NULL) {
        cout << n->data << " ";
        n = n->next;
    }
}
  
// Driver's code
int main()
{
    Node* head = NULL;
    Node* second = NULL;
    Node* third = NULL;
  
    // allocate 3 nodes in the heap
    head = new Node();
    second = new Node();
    third = new Node();
  
    head->data = 1; // assign data in first node
    head->next = second; // Link first node with second
  
    second->data = 2; // assign data to second node
    second->next = third;
  
    third->data = 3; // assign data to third node
    third->next = NULL;
  
    // Function call
    printList(head);
  
    return 0;
}
  
// This is code is contributed by rathbhupendra


Java




// A simple Java program for traversal of a linked list
  
class LinkedList {
  
    Node head; // head of list
  
    /* Linked list Node.  This inner class is made static so
       that main() can access it */
    static class Node {
  
        int data;
        Node next;
        Node(int d)
        {
            this.data = d;
            next = null;
        } // Constructor
    }
  
    /* This function prints contents of linked list starting
     * from head */
    public void printList()
    {
        Node n = head;
        while (n != null) {
            System.out.print(n.data + " ");
            n = n.next;
        }
    }
  
    // Driver's code
    public static void main(String[] args)
    {
        /* Start with the empty list. */
        LinkedList llist = new LinkedList();
  
        llist.head = new Node(1);
        Node second = new Node(2);
        Node third = new Node(3);
  
        llist.head.next = second; // Link first node with
                                  // the second node
        second.next
            = third; // Link second node with the third node
  
        // Function call
        llist.printList();
    }
}


Python3




# Python program for traversal of a linked list
# Node class
  
  
class 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 object
class LinkedList:
  
    # Function to initialize head
    def __init__(self):
        self.head = None
  
    # This function prints contents of linked list
    # starting from head
    def printList(self):
        temp = self.head
        while (temp):
            print(temp.data)
            temp = temp.next
  
  
# Code execution starts here
if __name__ == '__main__':
  
    # Start with the empty list
    llist = LinkedList()
  
    llist.head = Node(1)
    second = Node(2)
    third = Node(3)
  
    llist.head.next = second  # Link first node with second
    second.next = third  # Link second node with the third node
  
    llist.printList()


C#




// C# program for traversal of a linked list
  
using System;
  
public class LinkedList {
    Node head; // head of list
  
    /* Linked list Node. This inner
    class is made static so that
    main() can access it */
    public class Node {
        public int data;
        public Node next;
        public Node(int d)
        {
            data = d;
            next = null;
  
        } // Constructor
    }
  
    /* This function prints contents of
    linked list starting from head */
    public void printList()
    {
        Node n = head;
        while (n != null) {
            Console.Write(n.data + " ");
            n = n.next;
        }
    }
  
    // Driver's code
    public static void Main(String[] args)
    {
        /* Start with the empty list. */
        LinkedList llist = new LinkedList();
  
        llist.head = new Node(1);
        Node second = new Node(2);
        Node third = new Node(3);
  
        llist.head.next = second; // Link first node with
                                  // the second node
        second.next
            = third; // Link second node with the third node
  
        // Function call
        llist.printList();
    }
}
  
/* This code contributed by PrinciRaj1992 */


Javascript




// A simple javascript program for traversal of a linked list
    var head; // head of list
  
    /* Linked list Node.  This inner class is made  so that
       main() can access it */
    class Node {
        constructor(val) {
            this.data = val;
            this.next = null;
        }
    }
  
    /* This function prints contents of linked list starting from head */
     function printList()
    {
        var n = head;
        while (n != null) {
            document.write(n.data + " ");
            n = n.next;
        }
    }
  
    /* method to create a simple linked list with 3 nodes*/
      
      
        /* Start with the empty list. */
        
  
       var head = new Node(1);
        var second = new Node(2);
        var third = new Node(3);
  
        head.next = second; // Link first node with the second node
        second.next = third; // Link second node with the third node
  
        printList();
  
  
// This code contributed by gauravrajput1


Output

 1  2  3 

Time Complexity:

Time Complexity Worst Case Average Case
Search O(n) O(n)
Insert O(1) O(1)
Deletion O(1) O(1)

Auxiliary Space: O(N)


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