Insertion into circular singly linked list at beginning

There are two scenario in which a node can be inserted in circular singly linked list at beginning. Either the node will be inserted in an empty list or the node is to be inserted in an already filled list.

Firstly, allocate the memory space for the new node by using the malloc method of C language.

struct node *ptr = (struct node *)malloc(sizeof(struct node));

In the first scenario, the condition head == NULL will be true. Since, the list in which, we are inserting the node is a circular singly linked list, therefore the only node of the list (which is just inserted into the list) will point to itself only. We also need to make the head pointer point to this node. This will be done by using the following statements.

if(head == NULL)
		{
			head = ptr;
			ptr -> next = head;
		}

In the second scenario, the condition head == NULL will become false which means that the list contains at least one node. In this case, we need to traverse the list in order to reach the last node of the list. This will be done by using the following statement.

temp = head;
			while(temp->next != head)
				temp = temp->next;

At the end of the loop, the pointer temp would point to the last node of the list. Since, in a circular singly linked list, the last node of the list contains a pointer to the first node of the list. Therefore, we need to make the next pointer of the last node point to the head node of the list and the new node which is being inserted into the list will be the new head node of the list therefore the next pointer of temp will point to the new node ptr.

This will be done by using the following statements.

temp -> next = ptr; 

the next pointer of temp will point to the existing head node of the list.

ptr->next = head; 

Now, make the new node ptr, the new head node of the circular singly linked list.

head = ptr;

in this way, the node ptr has been inserted into the circular singly linked list at beginning.

Algorithm

• Step 1: IF PTR = NULL

  Write OVERFLOW
 Go to Step 11
 [END OF IF]

• Step 2: SET NEW_NODE = PTR
• Step 3: SET PTR = PTR -> NEXT
• Step 4: SET NEW_NODE -> DATA = VAL
• Step 5: SET TEMP = HEAD
• Step 6: Repeat Step 8 while TEMP -> NEXT != HEAD
• Step 7: SET TEMP = TEMP -> NEXT

[END OF LOOP]

• Step 8: SET NEW_NODE -> NEXT = HEAD
• Step 9: SET TEMP → NEXT = NEW_NODE
• Step 10: SET HEAD = NEW_NODE
• Step 11: EXIT

C Function

#include<stdio.h>
#include<stdlib.h>
void beg_insert(int);
struct node
{
	int data;
	struct node *next;
};
struct node *head;
void main ()
{
	int choice,item;
	do 
	{
		printf("\nEnter the item which you want to insert?\n");
		scanf("%d",&item);
		beg_insert(item);
		printf("\nPress 0 to insert more ?\n");
		scanf("%d",&choice);
	}while(choice == 0);
}
void beg_insert(int item)  
{  
      
    struct node *ptr = (struct node *)malloc(sizeof(struct node));  
    struct node *temp;
	if(ptr == NULL)  
    {  
        printf("\nOVERFLOW");  
    }  
    else   
    {  
        ptr -> data = item;  
        if(head == NULL)  
        {  
            head = ptr;  
            ptr -> next = head;  
        }  
        else   
        {     
            temp = head;  
            while(temp->next != head)  
                temp = temp->next;  
            ptr->next = head;   
            temp -> next = ptr;   
            head = ptr;  
        }   
	printf("\nNode Inserted\n");
    }  
              
}  
	

Output

	Enter the item which you want to insert?
12

Node Inserted

Press 0 to insert more ?
0

Enter the item which you want to insert?
90

Node Inserted

Press 0 to insert more ?
2