Linked List | Set 1 (Introduction)

• Difficulty Level : Easy
• Last Updated : 07 Jul, 2021

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.
 

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.  To complete your preparation from learning a language to DS Algo and many more,  please refer Complete Interview Preparation Course.

In case you wish to attend live classes with experts, please refer DSA Live Classes for Working Professionals and Competitive Programming Live for Students.

Why Linked List? 
Arrays can be used to store linear data of similar types, but arrays have the following limitations. 
1) 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. 
2) Inserting a new element in an array of elements is expensive because the room has to be created for the new elements and to create room existing elements have to be shifted. 
For example, in a system, if we maintain a sorted list of IDs in an array id[]. 
id[] = [1000, 1010, 1050, 2000, 2040]. 
And 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.
Advantages over arrays 
1) Dynamic size 
2) Ease of insertion/deletion
Drawbacks: 
1) Random access is not allowed. We have to access elements sequentially starting from the first node. So we cannot do binary search with linked lists efficiently with its default implementation. Read about it here. 
2) Extra memory space for a pointer is required with each element of the list. 
3) Not cache friendly. Since array elements are contiguous locations, there is locality of reference which is not there in case of linked lists.
Representation: 
A linked list is represented by a pointer to the first node of the linked list. The first node is called the head. If the linked list is empty, then the value of the head is NULL. 
Each node in a list consists of at least two parts: 
1) data 
2) Pointer (Or Reference) to the next 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
• C++
• Java
• Python
• C#

// A linked list node

struct Node {

    int data;

    struct Node* next;

};

First Simple Linked List in C Let us create a simple linked list with 3 nodes.
 

• C++
• C
• Java
• Python
• C#

// A simple CPP program to introduce

// a linked list

#include <bits/stdc++.h>

using namespace std;

class Node {

public:

    int data;

    Node* next;

};

// Program to create a simple linked

// list with 3 nodes

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

Linked List Traversal 
In the previous program, we have 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.
 

• C++
• C
• Java
• Python3
• 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 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;

    printList(head);

    return 0;

}

// This is code is contributed by rathbhupendra

Output: 

1  2  3