C - Linked List

A linked list is a linear data structure, in which the elements are stored in the form of a node. Each node contains two sub-elements. A data part that stores the value of the element and next part that stores the pointer to the next node as shown in the below image:

The first node also known as HEAD is always used as a reference to traverse the list. The last node points to NULL. Linked list can be visualized as a chain of nodes, where every node points to the next node.

Types of Linked List

The types of linked list are mentioned below:

Singly Linked List: can be traversed only in forward direction.
Doubly Linked List: can be traversed in forward and backward directions.
Circular Singly Linked List: Last element contains link to the first element as next.
Circular Doubly Linked List: Last element contains link to the first element as next and the first element contains link of the last element as previous.

Implementation of Singly Linked List

Representation:

In C, a node can be created using structure as shown below:

//node structure
struct Node {
  int data;
  struct Node* next;
};

Create a Singly Linked List

Let us create a simple singly linked list which contains three data nodes.

#include <stdio.h>
#include <stdlib.h>

//node structure
struct Node {
  int data;
  struct Node* next;
};

// test the code  
int main() {
  //create the head node with name MyList
  struct Node* MyList = NULL;

  //Add first node.
  struct Node* first;
  //allocate second node in the heap
  first = (struct Node*)malloc(sizeof(struct Node));  
  first->data = 10;
  first->next = NULL;
  //linking with head node
  MyList = first; 

  //Add second node.
  struct Node* second;
  //allocate second node in the heap
  second = (struct Node*)malloc(sizeof(struct Node));  
  second->data = 20;
  second->next = NULL;
  //linking with first node
  first->next = second;

  //Add third node.
  struct Node* third;
  //allocate third node in the heap
  third = (struct Node*)malloc(sizeof(struct Node));  
  third->data = 30;
  third->next = NULL;
  //linking with second node
  second->next = third; 

  return 0; 
}

Traverse a Singly Linked List

A singly linked list can be traversed using a temp node. Keep on moving the temp node to the next one and displaying its content. At the end of the list, the temp node will become NULL.

#include <stdio.h>
#include <stdlib.h>

//node structure
struct Node {
  int data;
  struct Node* next;
};

//display the content of the list
void PrintList(struct Node* head_ref) {
  struct Node* temp = head_ref;
  if(head_ref != NULL) {
    printf("The list contains: ");
    while (temp != NULL) {
      printf("%i ",temp->data);
      temp = temp->next;  
    }
    printf("\n");
  } else {
    printf("The list is empty.\n");
  }   
}

// test the code  
int main() {
  //create the head node with name MyList
  struct Node* MyList = NULL;

  //Add first node.
  struct Node* first;
  //allocate second node in the heap
  first = (struct Node*)malloc(sizeof(struct Node));  
  first->data = 10;
  first->next = NULL;
  //linking with head node
  MyList = first; 

  //Add second node.
  struct Node* second;
  //allocate second node in the heap
  second = (struct Node*)malloc(sizeof(struct Node));  
  second->data = 20;
  second->next = NULL;
  //linking with first node
  first->next = second;

  //Add third node.
  struct Node* third;
  //allocate third node in the heap
  third = (struct Node*)malloc(sizeof(struct Node));  
  third->data = 30;
  third->next = NULL;
  //linking with second node
  second->next = third; 

  //print the content of list
  PrintList(MyList);
  return 0; 
}

The above code will give the following output:

The list contains: 10 20 30