C++ - Delete first node by key of the Doubly Linked List

In this method, first node in the doubly linked list with specified key (value) is deleted. For example - if the given List is 10->20->30->10->20 and the first occurrence of 20 is deleted, the List becomes 10->30->10->20.

First, the head of the doubly linked list is checked for null value. If the head is not null and the value stored in it is equal to the key, make head next as head, delete previous head, and adjust previous of head. Else, traverse to the node previous to the node with value equal to key, and adjust links.

The function pop_first is created for this purpose. It is a 3-step process.

void pop_first(int key) {     
  Node* temp = head;
  //1. check if the head is not null
  if(temp != NULL) {
    
    //2. if head is not null and value stored at head
    //   is equal to the key, make head next as head,
    //   delete previous head, adjust previous of head
    if(temp->data == key) {
      Node* nodeToDelete = head;
      head = head->next;
      free(nodeToDelete);
      if(head != NULL)
        head->prev = NULL;
    } else {

      //3. Else, traverse to the node previous to the 
      //   node with value equal to key, and adjust links 
      while(temp->next != NULL) {
        if(temp->next->data == key) {
          Node* nodeToDelete = temp->next;
          temp->next = temp->next->next;
          if(temp->next != NULL)
            temp->next->prev = temp;          
          free(nodeToDelete);
          break; 
        }
        temp = temp->next;
      }
    }
  }
}

The below is a complete program that uses above discussed concept to delete first occurrence of the specified key (if exists) of the 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;
      } else {
        Node* temp = head;
        while(temp->next != NULL)
          temp = temp->next;
        temp->next = newNode;
        newNode->prev = temp;
      }    
    }

    //Delete first node by key
    void pop_first(int key) {     
      Node* temp = head;
      if(temp != NULL) {
        if(temp->data == key) {
          Node* nodeToDelete = head;
          head = head->next;
          free(nodeToDelete);
          if(head != NULL)
            head->prev = NULL;
        } else {
          while(temp->next != NULL) {
            if(temp->next->data == key) {
              Node* nodeToDelete = temp->next;
              temp->next = temp->next->next;
              if(temp->next != NULL)
                temp->next->prev = temp;          
              free(nodeToDelete);
              break; 
            }
            temp = temp->next;
          }
        }
      }
    } 

    //display the content of the list
    void PrintList() {
      Node* temp = head;
      if(temp != NULL) {
        cout<<"The list contains: ";
        while(temp != NULL) {
          cout<<temp->data<<" ";
          temp = temp->next;
        }
        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 the first occurrence of 20
  MyList.pop_first(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 20