C - Delete the first node of the Circular Singly Linked List

In this method, the first node of the circular singly linked list is deleted. For example - if the given list is 10->20->30->40 and the first node is deleted, the list becomes 20->30->40.

Deleting the first node of the circular singly linked list is very easy. If the list contains one node, delete the node. If the list contains more than one node, then create two nodes - temp and firstNode both pointing to the head. Using temp node, traverse to the last node of the list. Make next of head as the head node and next of last node as head. Finally, delete the first node.

Circular Singly Linked List - Delete First Node

The function pop_front is created for this purpose. It is a 4-step process.

void pop_front(struct Node** head_ref) {
  if(*head_ref != NULL) {
    
    //1. the list contains one node, delete
    //   make the head null
    if((*head_ref)->next == *head_ref) {
      *head_ref = NULL;
    } else {
      
      //2. if the list contains more than one node,
      //   create two nodes - temp and firstNode, both
      //   pointing to head node
      struct Node* temp = *head_ref;
      struct Node* firstNode = *head_ref;
      
      //3. using temp node, traverse to the last node
      while(temp->next != *head_ref) {
        temp = temp->next;
      }
      
      //4. Make head as next of head,
      //   Make next of last node as head,
      //   delete the firstNode
      *head_ref = (*head_ref)->next;
      temp->next = *head_ref; 
      free(firstNode); 
    } 
  }    
}

The below is a complete program that uses above discussed concept of deleting the first node of the circular singly linked list.

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

//node structure
struct Node {
  int data;
  struct Node* next;
};

//Add new element at the end of the list
void push_back(struct Node** head_ref, int newElement) {  
  struct Node *newNode, *temp;
  newNode = (struct Node*)malloc(sizeof(struct Node)); 
  newNode->data = newElement;  
  newNode->next = NULL;
  if(*head_ref == NULL) {
    *head_ref = newNode;
     newNode->next = *head_ref;
  } else {
    temp = *head_ref;
    while(temp->next != *head_ref) {
      temp = temp->next;
    }    
    temp->next = newNode;
    newNode->next = *head_ref;
  }
}

//Delete first node of the list
void pop_front(struct Node** head_ref) {
  if(*head_ref != NULL) {
    if((*head_ref)->next == *head_ref) {
      *head_ref = NULL;
    } else {
      struct Node* temp = *head_ref;
      struct Node* firstNode = *head_ref;
      while(temp->next != *head_ref) {
        temp = temp->next;
      }
      *head_ref = (*head_ref)->next;
      temp->next = *head_ref; 
      free(firstNode); 
    } 
  }    
}

//display the content of the list
void PrintList(struct Node* head_ref) {
  struct Node* temp = head_ref;
  if(head_ref != NULL) {
    printf("The list contains: ");
    while (1) {
      printf("%i ",temp->data);
      temp = temp->next;
      if(temp == head_ref)
        break;    
    }
    printf("\n");
  } else {
    printf("The list is empty.\n");
  }   
}

// test the code 
int main() {
  struct Node* MyList = NULL;

  //Add four elements in the list.
  push_back(&MyList, 10);
  push_back(&MyList, 20);
  push_back(&MyList, 30);
  push_back(&MyList, 40);
  PrintList(MyList);

  //Delete the first node
  pop_front(&MyList);
  PrintList(MyList);

  return 0; 
}

The above code will give the following output:

The list contains: 10 20 30 40
The list contains: 20 30 40