circular queue in linked list 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 Linked List in c++

#include <iostream>
using namespace std;

#define  NULL  0


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

struct  node  *first=NULL ;
struct  node  *last=NULL ;

void  create()
{
  int  i , n ;
  struct  node  *pnode , *p ;

  printf("Enter the number of nodes required:\n") ;
  scanf("%d",&n) ;

  printf("Enter the data value of each node:\n") ;
  for(i=1 ; i<=n ; i++)
  {
    pnode=(struct node*)malloc(sizeof(struct node)) ;
    if(pnode==NULL)
    {
      printf("Memory overflow. Unable to create.\n") ;
      return ;
    }

    scanf("%d",&pnode->data) ;

    if(first==NULL)
	 first=last=pnode ;
    else
    {
	  last->next=pnode ;
	  last=pnode ;    /* last keeps track of last node */
    }

    last->next=first ;
  }
}

/* This function will delete a node with value k from the Linked List if such a node exists */
void  deletenode(int  k)
{
  struct  node  *p , *follow ;

  /* searching the required node */
  p=first ;
  follow=NULL ;
  while(follow!=last)
  {
    if(p->data==k)
	  break ;
    follow=p ;
    p=p->next ;
  }

  if(follow==last)
    printf("Required node not found.\n") ;
  else
  {
    if(p==first&&p==last)  /* deleting the one and the only node */
	  first=last=NULL ;
    else if(p==first)       /* deleting the first node */
    {
      first=first->next ;
      last->next=first ;
    }
    else if(p==last)      /* deleting the last node */
    {
      last=follow ;
      last->next=first ;
    }
    else		/* deleting any other node */
      follow->next=p->next ;

    free(p) ;
  }
}

/* This function will go through all the nodes of Linked List exactly once and will display data value of each node */
void  traverse()
{
  struct  node  *p , *follow ;
  if(first==NULL)
    printf("Circularly Linked List Empty") ;
  else
  {
    printf("Circularly Linked List is as shown: \n") ;

    p=first ;
    follow = NULL ;
    while(follow!=last)
    {
      printf("%d " , p->data) ;
      follow=p ;
      p=p->next ;
    }

    printf("\n") ;
  }
}

int main()
{
  int  x , k , ch ;
 
  do
  {
    printf("\n Menu: \n") ;
    printf("1:Create Linked List \n") ;
    printf("2:Delete Node \n") ;
    printf("3:Traverse \n") ;
    printf("4:Exit \n") ;

    printf("\nEnter your choice: ") ;
    scanf("%d",&ch) ;

    switch(ch)
    {
      case 1:
      create() ;
      break ;

      case 2:
      printf("Enter the data value of the node to be deleted: ") ;
      scanf("%d",&k) ;
      deletenode(k) ;
      break ;

      case 3:
      traverse() ;
      break ;

      case 4:
      break ;
    }
 }
 while(ch!=4) ;

 return 0;
}