C++ - Delete the last node of the Doubly Linked List

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

Deleting the last node of the Doubly Linked List involves checking the head for empty. If it is not empty, then check the head next for empty. If the head next is empty, then release the head, else traverse to the second last node of the list. Then, link the next of second last node to NULL and delete the last node.

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

void pop_back() {
  if(head != NULL) {
    
    //1. if head in not null and next of head
    //   is null, release the head
    if(head->next == NULL) {
      head = NULL;
    } else {
      
      //2. Else, traverse to the second last 
      //   element of the list
      Node* temp = head;
      while(temp->next->next != NULL)
        temp = temp->next;
      
      //3. Change the next of the second 
      //   last node to null and delete the
      //   last node
      Node* lastNode = temp->next;
      temp->next = NULL;
      free(lastNode); 
    }
  }
}

The below is a complete program that uses above discussed concept of deleting the last node 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 last node of the list
    void pop_back() {
      if(head != NULL) {
        if(head->next == NULL) {
          head = NULL;
        } else {
          Node* temp = head;
          while(temp->next->next != NULL)
            temp = temp->next;
          Node* lastNode = temp->next;
          temp->next = NULL;
          free(lastNode); 
        }
      }
    }

    //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 four elements in the list.
  MyList.push_back(10);
  MyList.push_back(20);
  MyList.push_back(30);
  MyList.push_back(40);  
  MyList.PrintList();
  
  //Delete the last node
  MyList.pop_back();
  MyList.PrintList();  
  
  return 0; 
}

The above code will give the following output:

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