Convert a Singly Linked List to Circular Linked List

Write a C program to convert a singly linked list to a circular linked list.

How to make a circular linked list from singly linked list.

Given a singly linked list, we have to convert it to a circular linked list. We have to set the next pointer of tail node to point back to head node to complete a cycle.

Algorithm to convert singly linked list to Circular linked list
Let "head" be the head pointer of given linked list. Singly linked list is a linear data structure, which can be traversed from first node(head node) till last node(tail node) in one direction. Tail node of linked list points to NULL, Hence it is a dead end. We cannot traverse beyond tail node. To convert a singly linked list to circular linked list, we will set next pointer of tail node to head pointer.

Create a copy of head pointer, let's say "temp".
Using a loop, traverse linked list till tail node(last node) using temp pointer.
Now set the next pointer of tail node to head node. (temp->next = head;)

In this program, we will use a user defined function "convertToCircularLL" which takes head node of singly linked list as input and converts it to a Circular linked list by implementing above algorithm.

void convertToCircularLL(struct node *head){
    /* Input Validation */
    if(head == NULL){
        printf("Error : Invalid Input !!!!\n");
        return INT_MIN;
    }
    struct node *temp = head;
    while(temp->next != NULL){
        temp = temp->next;
    }
    
    temp->next = head;
}

C program to convert a singly linked list to circular linked list

#include <stdio.h>
#include <stdlib.h>
 
/* A structure of linked list node */
struct node {
  int data;
  struct node *next;
} *head;

void initialize(){
    head = NULL;
}

/* 
Given a Inserts a node in front of a singly linked list. 
*/
void insert(int num) {
    /* Create a new Linked List node */
    struct node* newNode = (struct node*) malloc(sizeof(struct node));
    newNode->data  = num;
    /* Next pointer of new node will point to head node of linked list  */
    newNode->next = head;
    /* make new node as new head of linked list */
    head = newNode;
    printf("Inserted Element : %d\n", num);
}

void convertToCircularLL(struct node *head){
    /* Input Validation */
    if(head == NULL){
        printf("Error : Invalid Input !!!!\n");
        return INT_MIN;
    }
    struct node *temp = head;
    while(temp->next != NULL){
        temp = temp->next;
    }
    
    temp->next = head;
}

/*
Prints a linked list from head node till tail node 
*/
void printCircularLinkedList(struct node *head) {
    struct node *temp = head;
    do {
        printf("%d ", temp->data);
        temp = temp->next;
    } while(temp != head);
}
 
int main() {
    initialize();
    /* Creating a linked List*/
    insert(1);  
    insert(2); 
    insert(3); 
    insert(4);
    insert(5);
    
    convertToCircularLL(head);
    
    /* Printing Circular Linked List */
    printCircularLinkedList(head);
    return 0;
}

Output

Inserted Element : 1
Inserted Element : 2
Inserted Element : 3
Inserted Element : 4
Inserted Element : 5
5 4 3 2 1