Data Structures - Doubly Linked List Other Related Topics
Data Structures - Doubly Linked List Other Related Topics

Doubly Linked List - Delete the last node

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.

Doubly Linked List - Delete 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); 
    }
  }
}

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

def pop_back(self):
  if(self.head != None):
    
    #1. if head in not null and next of head
    #   is null, release the head
    if(self.head.next == None):
      self.head = None
    else:
      
      #2. Else, traverse to the second last 
      #   element of the list
      temp = self.head
      while(temp.next.next != None):
        temp = temp.next
      
      #3. Change the next of the second 
      #   last node to null and delete the
      #   last node
      lastNode = temp.next
      temp.next = None
      lastNode = None

void pop_back() {
  if(this.head != null) {
    
    //1. if head in not null and next of head
    //   is null, release the head
    if(this.head.next == null) {
      this.head = null;
    } else {
      
      //2. Else, traverse to the second last 
      //   element of the list
      Node temp = new Node();
      temp = this.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; 
      lastNode = null;
    }
  }
}

public void pop_back() {
  if(this.head != null) {
    
    //1. if head in not null and next of head
    //   is null, release the head
    if(this.head.next == null) {
      this.head = null;
    } else {
      
      //2. Else, traverse to the second last 
      //   element of the list
      Node temp = new Node();
      temp = this.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; 
      lastNode = null;
    }
  }
}

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

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<<"\n";
      } 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 

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

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

//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;
  newNode->prev = NULL;
  if(*head_ref == NULL) {
    *head_ref = newNode;
  } else {
    temp = *head_ref;
    while(temp->next != NULL) {
      temp = temp->next;
    }    
    temp->next = newNode;
    newNode->prev = temp;
  }
}

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

//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 (temp != NULL) {
      printf("%i ",temp->data);
      temp = temp->next;  
    }
    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 last node
  pop_back(&MyList);
  PrintList(MyList);

  return 0; 
}

The above code will give the following output: 

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

# node structure
class Node:
  def __init__(self, data):
    self.data = data
    self.next = None
    self.prev = None

#class Linked List
class LinkedList:
  def __init__(self):
    self.head = None

  #Add new element at the end of the list
  def push_back(self, newElement):
    newNode = Node(newElement)
    if(self.head == None):
      self.head = newNode
      return
    else:
      temp = self.head
      while(temp.next != None):
        temp = temp.next
      temp.next = newNode
      newNode.prev = temp

  #Delete last node of the list
  def pop_back(self):
    if(self.head != None):
      if(self.head.next == None):
        self.head = None
      else:
        temp = self.head
        while(temp.next.next != None):
          temp = temp.next
        lastNode = temp.next
        temp.next = None
        lastNode = None

  #display the content of the list
  def PrintList(self):
    temp = self.head
    if(temp != None):
      print("The list contains:", end=" ")
      while (temp != None):
        print(temp.data, end=" ")
        temp = temp.next
      print()
    else:
      print("The list is empty.")

# test the code                  
MyList = LinkedList()

#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()

The above code will give the following output: 

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

//node structure
class Node {
    int data;
    Node next;
    Node prev;
};

class LinkedList {
  Node head;

  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 = new 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(this.head != null) {
      if(this.head.next == null) {
        this.head = null;
      } else {
        Node temp = new Node();
        temp = this.head;
        while(temp.next.next != null)
          temp = temp.next;
        Node lastNode = temp.next;
        temp.next = null; 
        lastNode = null;
      }
    }
  }

  //display the content of the list
  void PrintList() {
    Node temp = new Node();
    temp = this.head;
    if(temp != null) {
      System.out.print("The list contains: ");
      while(temp != null) {
        System.out.print(temp.data + " ");
        temp = temp.next;
      }
      System.out.println();
    } else {
      System.out.println("The list is empty.");
    }
  }    
};

// test the code 
public class Implementation {
  public static void main(String[] args) {
    LinkedList MyList = new LinkedList();

    //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();  
  }
}

The above code will give the following output: 

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

using System;

//node structure
class Node {
  public int data;
  public Node next;
  public Node prev;
};

class LinkedList {
  Node head;

  public LinkedList(){
    head = null;
  }
  
  //Add new element at the end of the list
  public 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 = new Node();
      temp = head;
      while(temp.next != null)
        temp = temp.next;
      temp.next = newNode;
      newNode.prev = temp;
    }    
  }

  //Delete last node of the list
  public void pop_back() {
    if(this.head != null) {
      if(this.head.next == null) {
        this.head = null;
      } else {
        Node temp = new Node();
        temp = this.head;
        while(temp.next.next != null)
          temp = temp.next;
        Node lastNode = temp.next;
        temp.next = null; 
        lastNode = null;
      }
    }
  }

  //display the content of the list
  public void PrintList() {
    Node temp = new Node();
    temp = this.head;
    if(temp != null) {
      Console.Write("The list contains: ");
      while(temp != null) {
        Console.Write(temp.data + " ");
        temp = temp.next;
      }
      Console.WriteLine();
    } else {
      Console.WriteLine("The list is empty.");
    }
  }     
};

// test the code
class Implementation {  
  static void Main(string[] args) {
    LinkedList MyList = new LinkedList();

    //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();    
  }
}

The above code will give the following output: 

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

<?php
//node structure
class Node {
  public $data;
  public $next;
  public $prev;
}

class LinkedList {
  public $head;

  public function __construct(){
    $this->head = null;
  }
  
  //Add new element at the end of the list
  public function push_back($newElement) {
    $newNode = new Node();
    $newNode->data = $newElement;
    $newNode->next = null;
    $newNode->prev = null; 
    if($this->head == null) {
      $this->head = $newNode;
    } else {
      $temp = new Node();
      $temp = $this->head;
      while($temp->next != null) {
        $temp = $temp->next;
      }
      $temp->next = $newNode;
      $newNode->prev = $temp;
    }    
  }

  //Delete last node of the list
  public function pop_back() {
    if($this->head != null) {
      if($this->head->next == null) {
        $this->head = null;
      } else {
        $temp = new Node();
        $temp = $this->head;
        while($temp->next->next != null)
          $temp = $temp->next;
        $lastNode = $temp->next;
        $temp->next = null; 
        $lastNode = null;
      }
    }
  }
  
  //display the content of the list
  public function PrintList() {
    $temp = new Node();
    $temp = $this->head;
    if($temp != null) {
      echo "The list contains: ";
      while($temp != null) {
        echo $temp->data." ";
        $temp = $temp->next;
      }
      echo "\n";
    } else {
      echo "The list is empty.\n";
    }
  }   
};

// test the code  
$MyList = new LinkedList();

//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(); 
?>

The above code will give the following output: 

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