C++ - Delete all nodes by key of the Circular Doubly Linked List

In this method, all nodes in the circular doubly linked list with specified key (value) is deleted. For example - if the given list is 10->20->30->10->20 and all occurrences of 20 are deleted, the list becomes 10->30->10.

If the head of the circular doubly linked list is not null and the value stored at head is equal to key, then adjust the head as head next, delete previous head and adjust links. Keep on doing the same process until the new head becomes null or not equal to the key. After that create a temp node to traverse through the list and delete those nodes with value equal to the key and adjust links accordingly.

The function pop_all is created for this purpose. It is a 2-step process.

void pop_all(int key) {     
  Node* nodeToDelete;
  Node* temp;
  
  //1. if the head is not null and value stored at
  //   head is equal to key, keep on adjusting the
  //   head as head next and deleting previous head
  //   until new head becomes null or not equal to key
  while(head != NULL && head->data == key) {

    if(head->next == head) {
      head = NULL;
    } else {
      nodeToDelete = head;
      temp = head;
      // traverse to the last element of
      // the list to adjust links
      while(temp->next != head) {
        temp = temp->next;
      } 
      head = head->next;
      temp->next = head; 
      head->prev = temp;  
      free(nodeToDelete);
    }
  }

  //2. create a temp node to traverse through the
  //   list and delete nodes with value equal to 
  //   key and adjust links accordingly
  temp = head;        
  if(temp != NULL) {
    while(temp->next != head) {
      if(temp->next->data == key) {
        nodeToDelete = temp->next;
        temp->next = temp->next->next;
        temp->next->prev = temp;
        free(nodeToDelete);
      } else {
        temp = temp->next;
      }
    }
  }
}

The below is a complete program that uses above discussed concept to delete all occurrences of the specified key (if exists) 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 end of the list
    void push_back(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->next = head;
        newNode->prev = temp;
        head->prev = newNode;
      }    
    }

    //Delete all nodes by key
    void pop_all(int key) {     
      Node* nodeToDelete;
      Node* temp;
      
      while(head != NULL && head->data == key) {

        if(head->next == head) {
          head = NULL;
        } else {
          nodeToDelete = head;
          temp = head;
          while(temp->next != head) {
            temp = temp->next;
          } 
          head = head->next;
          temp->next = head; 
          head->prev = temp;  
          free(nodeToDelete);
        }
      }

      temp = head;        
      if(temp != NULL) {
        while(temp->next != head) {
          if(temp->next->data == key) {
            nodeToDelete = temp->next;
            temp->next = temp->next->next;
            temp->next->prev = temp;
            free(nodeToDelete);
          } else {
            temp = temp->next;
          }
        }
      }
    } 

    //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 five elements at the end of the list.
  MyList.push_back(10);
  MyList.push_back(20);
  MyList.push_back(30);
  MyList.push_back(10);
  MyList.push_back(20);
  MyList.PrintList();

  //Delete all occurrences of 20
  MyList.pop_all(20);
  MyList.PrintList();

  return 0; 
}

The above code will give the following output:

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