how to make a circularly linked list C++ code example

Example 1: circular linked list in c

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

struct node {
   int data;
   int key;
	
   struct node *next;
};

struct node *head = NULL;
struct node *current = NULL;

bool isEmpty() {
   return head == NULL;
}

int length() {
   int length = 0;

   //if list is empty
   if(head == NULL) {
      return 0;
   }

   current = head->next;

   while(current != head) {
      length++;
      current = current->next;   
   }
	
   return length;
}

//insert link at the first location
void insertFirst(int key, int data) {

   //create a link
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;
	
   if (isEmpty()) {
      head = link;
      head->next = head;
   } else {
      //point it to old first node
      link->next = head;
		
      //point first to new first node
      head = link;
   }    
}

//delete first item
struct node * deleteFirst() {

   //save reference to first link
   struct node *tempLink = head;
	
   if(head->next == head) {  
      head = NULL;
      return tempLink;
   }     

   //mark next to first link as first 
   head = head->next;
	
   //return the deleted link
   return tempLink;
}

//display the list
void printList() {

   struct node *ptr = head;
   printf("\n[ ");
	
   //start from the beginning
   if(head != NULL) {
	
      while(ptr->next != ptr) {     
         printf("(%d,%d) ",ptr->key,ptr->data);
         ptr = ptr->next;
      }
   }
	
   printf(" ]");
}

void main() {
   insertFirst(1,10);
   insertFirst(2,20);
   insertFirst(3,30);
   insertFirst(4,1);
   insertFirst(5,40);
   insertFirst(6,56); 

   printf("Original List: "); 
	
   //print list
   printList();

   while(!isEmpty()) {            
      struct node *temp = deleteFirst();
      printf("\nDeleted value:");  
      printf("(%d,%d) ",temp->key,temp->data);
   }   
	
   printf("\nList after deleting all items: ");
   printList();   
}

Example 2: circular queue using linked list in c++

#include<iostream>

#define SIZE 100

using namespace std;

class node
{
public:
    node()
    {
        next = NULL;
    }
  int data;
  node *next;
}*front=NULL,*rear=NULL,*n,*temp,*temp1;

class cqueue
{
public:
    void insertion();
    void deletion();
    void display();
};

int main()
{
    cqueue cqobj;
  int ch;
  do
  {
     cout<<"\n\n\tMain Menu";
     cout<<"\n##########################";
     cout<<"\n1. Insert\n2. Delete\n3. Display\n4. Exit\n\nEnter Your Choice: ";
     cin>>ch;
     switch(ch)
     {
        case 1:
          cqobj.insertion();
          cqobj.display();
          break;
        case 2:
          cqobj.deletion();
          break;
        case 3:
          cqobj.display();
          break;
        case 4:
          break;
        default:
          cout<<"\n\nWrong Choice!!! Try Again.";
     }
  }while(ch!=4);
  return 0;
}

void cqueue::insertion()
{
  n=new node[sizeof(node)];
  cout<<"\nEnter the Element: ";
  cin>>n->data;
  if(front==NULL)
  {
      front=n;
  }
  else
  {
      rear->next=n;
  }
  rear=n;
  rear->next=front;
}

void cqueue::deletion()
{
  int x;
  temp=front;
  if(front==NULL)
  {
      cout<<"\nCircular Queue Empty!!!";
  }
  else
  {
     if(front==rear)
     {
       x=front->data;
       delete(temp);
       front=NULL;
       rear=NULL;
     }
     else
     {
        x=temp->data;
        front=front->next;
        rear->next=front;
        delete(temp);
     }
     cout<<"\nElement "<<x<<" is Deleted";
     display();
  }
}

void cqueue::display()
{
  temp=front;
  temp1=NULL;
  if(front==NULL)
  {
    cout<<"\n\nCircular Queue Empty!!!";
  }
  else
  {
    cout<<"\n\nCircular Queue Elements are:\n\n";
    while(temp!=temp1)
    {
       cout<<temp->data<<"  ";
       temp=temp->next;
       temp1=front;
    }
  }
}