C++ - Insert a new node at the start of the Circular Doubly Linked List

In this method, a new node is inserted at the start of the circular doubly linked list. For example - if the given List is 10->20->30 and a new element 100 is added at the start, the List becomes 100->10->20->30.

Inserting a new node at the start of the circular doubly linked list is very easy. First, a new node with given element is created. It is then added at the start of the list by linking the head node and last node to the new node.

Circular Doubly Linked List - Add Node At Start

The function push_front is created for this purpose. It is a 6-step process.

void push_front(int newElement) {
  
  //1. allocate node
  Node* newNode = new Node();
  
  //2. assign data element
  newNode->data = newElement;
  
  //3. assign null to the next and prev
  //   of the new node
  newNode->next = NULL;
  newNode->prev = NULL; 
  
  //4. Check the list is empty or not,
  //   if empty make the new node as head 
  if(head == NULL) {
    head = newNode;
    newNode->next = head;
    newNode->prev = head;
  } else {
    
    //5. Else, traverse to the last node
    Node* temp = head;
    while(temp->next != head)
      temp = temp->next;
    
    //6. Adjust the links
    temp->next = newNode;
    newNode->prev = temp;
    newNode->next = head;
    head->prev = newNode;
    head = newNode;
  }    
}

The below is a complete program that uses above discussed concept to insert new node at the start of the circular doubly linked list.

#include <iostream>
using namespace std;

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

class LinkedList {
  private:
    Node* head;
  public:
    LinkedList(){
      head = NULL;
    }
 
    //Add new element at the start of the list
    void push_front(int newElement) {
      Node* newNode = new Node();
      newNode->data = newElement;
      newNode->next = NULL;
      newNode->prev = NULL; 
      if(head == NULL) {
        head = newNode;
        newNode->next = head;
        newNode->prev = head;
      } else {
        Node* temp = head;
        while(temp->next != head)
          temp = temp->next;
        temp->next = newNode;
        newNode->prev = temp;
        newNode->next = head;
        head->prev = newNode;
        head = newNode;
      }    
    }

    //display the content of the list
    void PrintList() {
      Node* temp = head;
      if(temp != NULL) {
        cout<<"The list contains: ";
        while(true) {
          cout<<temp->data<<" ";
          temp = temp->next;
          if(temp == head) 
            break;
        }
        cout<<endl;
      } else {
        cout<<"The list is empty.\n";
      }
    }     
};

// test the code 
int main() {
  LinkedList MyList;

  //Add three elements at the start of the list.
  MyList.push_front(10);
  MyList.push_front(20);
  MyList.push_front(30);
  MyList.PrintList();
  
  return 0; 
}

The above code will give the following output:

The list contains: 30 20 10