Linked List | Set 1 (Introduction)

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.

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.

// 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.

// A simple C program to introduce 

// a linked list 

#include <stdio.h> 

#include <stdlib.h> 

struct Node { 

int data; 

struct Node* next; 

}; 

// Program to create a simple linked 

// list with 3 nodes 

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; 

} 

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.

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

// 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; 

} 

} 

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; 

printList(head); 

return 0; 

}

Output:

    1 2 3