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Insert node at first position in circular doubly link list

Now we will see how to insert a new node at first position in circular doubly link list.

Algorithm for insertion at first position in circular doubly link list:

In this algorithm, START is pointer to first node and PTR is node to be inserted. The step are as follows:
  • Create new node PTR
  • Set the info field of PTR
  • If list is empty i.e. START == NULL
    • Set START = PTR
    • Set PTR->NEXT = PTR
    • Set PTR->PREV = PTR
  • Else
    • Set PTR->NEXT = START
    • Set PTR->PREV = START->PREV
    • Set START->PREV->NEXT = PTR
    • Set START->PREV = PTR
    • Set START = PTR
  • End If;

Function to insert node at first position in circular doubly link list:


void insertAtFirst(NODE **start, int info){
 NODE *ptr = (NODE*) malloc(sizeof(NODE)); //new node to insert
 ptr->info = info; //set info field
 if(*start == NULL){ //empty list
  *start = ptr;
  ptr->next = ptr;
  ptr->prev = ptr;
 }
 else{
  ptr->next = *start; //ptr's next points to first node
  ptr->prev = (*start)->prev; // ptr's prev points to last node
  (*start)->prev->next = ptr; //last node's next point to ptr
  (*start)->prev = ptr; //first node's prev points to ptr
  *start = ptr; //now ptr is first node
 }
}

Program to insert at first position in the circular doubly link list:


#include <stdio.h>
#include <malloc.h>

typedef struct node{
 struct node *prev;
 int info;
 struct node *next;
} NODE;

void insertAtFirst(NODE **, int);
void traverse(NODE **);

int main(){
 NODE *start = NULL;
 insertAtFirst(&start, 3);
 insertAtFirst(&start, 24);
 insertAtFirst(&start, 4);
 traverse(&start);
 return 0;
}

void insertAtFirst(NODE **start, int info){
 NODE *ptr = (NODE*) malloc(sizeof(NODE)); //new node to insert
 ptr->info = info; //set info field
 if(*start == NULL){ //empty list
  *start = ptr;
  ptr->next = ptr;
  ptr->prev = ptr;
 }
 else{
  ptr->next = *start; //ptr's next points to first node
  ptr->prev = (*start)->prev; // ptr's prev points to last node
  (*start)->prev->next = ptr; //last node's next point to ptr
  (*start)->prev = ptr; //first node's prev points to ptr
  *start = ptr; //now ptr is first node
 }
}

void traverse(NODE **start){
 NODE *ptr = *start;
 if(ptr == NULL){
  printf("\nEmpty list\n");
  return;
 }
 while( ptr->next != *start ){
  printf("%d  ",ptr->info);
  ptr = ptr->next;
 }
 printf("%d\n",ptr->info);
}



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