Data Structures - Doubly Linked List Other Related Topics
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Doubly Linked List - Delete odd nodes



Deleting odd nodes of a doubly linked list requires traverse through the list and deleting odd nodes one by one. If the list is not null then release the head and make next of head as new head. If the new head is not null then make the next of head as null and create two nodes - evenNode and oddNode. Make evenNode to the first even node of list and oddNode to second odd node of the list (3rd node in the original list). If both are not null, delete the oddNode and adjust links. Move both nodes to next set of even-odd nodes. Repeat the process till the any or both nodes become null.

The function deleteOddNodes is created for this purpose. It is a 5-step process.

void deleteOddNodes() {
  if(head != NULL) {
    
    //1. if head is not null, make next of head as
    //   new head and delete previous head
    Node* temp = head;
    head = head->next; 
    free(temp);   
    if(head != NULL) {

      //2. if the new head is not make prev of head as
      //   null and create nodes - evenNode and oddNode
      head->prev = NULL;
      Node* evenNode = head;
      Node* oddNode = head->next; 

      while(evenNode != NULL && oddNode != NULL) {
        
        //3. while evenNode and oddNode are not null
        //   make next of evenNode as next of oddNode 
        //   and free oddNode   
        evenNode->next = oddNode->next;
        free(oddNode);

        //4. and make temp as evenNode and evenNode as 
        //   next of evenNode
        temp = evenNode;
        evenNode = evenNode->next;
        
        //5. Update prev link, evenNode and oddNode
        if(evenNode != NULL) {
          evenNode->prev = temp;
          oddNode = evenNode->next;
        }
      }
    }
  }
} 
void deleteOddNodes(struct Node** head_ref) {
  if(*head_ref != NULL) {
    
    //1. if head is not null, make next of head as
    //   new head and delete previous head
    struct Node* temp = *head_ref;
    *head_ref = (*head_ref)->next;
    free(temp);
    if(*head_ref != NULL) {
      
      //2. if the new head is not make prev of head as
      //   null and create nodes - evenNode and oddNode
      (*head_ref)->prev = NULL;
      struct Node* evenNode = *head_ref;
      struct Node* oddNode = (*head_ref)->next;

      while(evenNode != NULL && oddNode != NULL) {
        
        //3. while evenNode and oddNode are not null
        //   make next of evenNode as next of oddNode 
        //   and free oddNode   
        evenNode->next = oddNode->next;
        free(oddNode);

        //4. and make temp as evenNode and evenNode as 
        //   next of evenNode
        temp = evenNode;
        evenNode = evenNode->next;
        
        //5. Update prev link, evenNode and oddNode
        if(evenNode != NULL) {
          evenNode->prev = temp;
          oddNode = evenNode->next;
        }
      }
    }
  }   
}     
def deleteOddNodes(self):  
  if (self.head != None):
    
    #1. if head is not null, make next of head as
    #   new head and delete previous head
    temp = self.head
    self.head = self.head.next
    temp = None
    if (self.head != None):
      
      #2. if the new head is not make prev of head as
      #   null and create nodes - evenNode and oddNode
      self.head.prev = None
      evenNode = self.head
      oddNode = self.head.next 

      while(evenNode != None and oddNode != None):
        
        #3. while evenNode and oddNode are not null
        #   make next of evenNode as next of oddNode
        #   and free oddNode   
        evenNode.next = oddNode.next
        oddNode = None

        #4. and make temp as evenNode and evenNode as 
        #   next of evenNode
        temp = evenNode
        evenNode = evenNode.next
        
        #5. Update prev link, evenNode and oddNode
        if(evenNode != None):
          evenNode.prev = temp
          oddNode = evenNode.next
void deleteOddNodes() {
  if(this.head != null) {
    
    //1. if head is not null, make next of head as
    //   new head and delete previous head
    Node temp = this.head;
    this.head = this.head.next;
    temp = null;
    if(this.head != null) {
      
      //2. if the new head is not make prev of head as
      //   null and create nodes - evenNode and oddNode
      this.head.prev = null;
      Node evenNode = this.head;
      Node oddNode = this.head.next; 

      while(evenNode != null && oddNode != null) {
        
        //3. while evenNode and oddNode are not null
        //   make next of evenNode as next of oddNode 
        //   and free oddNode  
        evenNode.next = oddNode.next;
        oddNode = null;

        //4. and make temp as evenNode and evenNode as 
        //   next of evenNode
        temp = evenNode;
        evenNode = evenNode.next;
        
        //5. Update prev link, evenNode and oddNode
        if(evenNode != null) {
          evenNode.prev = temp;
          oddNode = evenNode.next;
        }
      }
    }
  }
}         
public void deleteOddNodes() {
  if(this.head != null) {
    
    //1. if head is not null, make next of head as
    //   new head and delete previous head
    Node temp = this.head;
    this.head = this.head.next;
    temp = null;
    if(this.head != null) {  
      
      //2. if the new head is not make prev of head as
      //   null and create nodes - evenNode and oddNode
      this.head.prev = null;
      Node evenNode = this.head;
      Node oddNode = this.head.next; 

      while(evenNode != null && oddNode != null) {
        
        //3. while evenNode and oddNode are not null
        //   make next of evenNode as next of oddNode 
        //   and free oddNode  
        evenNode.next = oddNode.next;
        oddNode = null;

        //4. and make temp as evenNode and evenNode as 
        //   next of evenNode
        temp = evenNode;
        evenNode = evenNode.next;
        
        //5. Update prev link, evenNode and oddNode
        if(evenNode != null) {
          evenNode.prev = temp;
          oddNode = evenNode.next;
        }
      }
    }
  }
}        
public function deleteOddNodes() {
  if($this->head != null) {
    
    //1. if head is not null, make next of head as
    //   new head and delete previous head
    $temp = $this->head;
    $this->head = $this->head->next;
    $temp = null;
    if($this->head != null) { 
      
      //2. if the new head is not make prev of head as
      //   null and create nodes - evenNode and oddNode
      $this->head->prev = null;
      $evenNode = $this->head;
      $oddNode = $this->head->next; 

      while($evenNode != null && $oddNode != null) {
        
        //3. while evenNode and oddNode are not null
        //   make next of evenNode as next of oddNode 
        //   and free oddNode  
        $evenNode->next = $oddNode->next;
        $oddNode = null;

        //4. and make temp as evenNode and evenNode as 
        //   next of evenNode
        $temp = $evenNode;
        $evenNode = $evenNode->next;
        
        //5. Update prev link, evenNode and oddNode
        if($evenNode != null) {
          $evenNode->prev = $temp;
          $oddNode = $evenNode->next;
        }
      }
    }
  }
}      

The below is a complete program that uses above discussed concept of deleting odd nodes of a 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 odd nodes of the list
  	void deleteOddNodes() {
  	  if(head != NULL) {
  	    Node* temp = head;
  	    head = head->next; 
  	    free(temp);   
  	    if(head != NULL) {
  	      head->prev = NULL;
  	      Node* evenNode = head;
  	      Node* oddNode = head->next; 
  	      while(evenNode != NULL && oddNode != NULL) {
  	        evenNode->next = oddNode->next;
  	        free(oddNode);
  	        temp = evenNode;
  	        evenNode = evenNode->next;
  	        if(evenNode != NULL) {
  	          evenNode->prev = temp;
  	          oddNode = evenNode->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<<"\n";
      } else {
        cout<<"The list is empty.\n";
      }
    }   
};

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

  //Add five elements in the list.
  MyList.push_back(10);
  MyList.push_back(20);
  MyList.push_back(30);
  MyList.push_back(40);
  MyList.push_back(50);

  //Display the content of the list.
  MyList.PrintList();

  //delete odd nodes of the list
  MyList.deleteOddNodes();

  cout<<"After deleting odd nodes.\n";
  //Display the content of the list.
  MyList.PrintList();
  
  return 0; 
}

The above code will give the following output:

The list contains: 10 20 30 40 50 
After deleting odd nodes.
The list contains: 20 40
#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 odd nodes of the list
void deleteOddNodes(struct Node** head_ref) {
  if(*head_ref != NULL) {
    struct Node* temp = *head_ref;
    *head_ref = (*head_ref)->next;
    free(temp);
    if(*head_ref != NULL) {
      (*head_ref)->prev = NULL;
      struct Node* evenNode = *head_ref;
      struct Node* oddNode = (*head_ref)->next;
      while(evenNode != NULL && oddNode != NULL) {
        evenNode->next = oddNode->next;
        free(oddNode);
        temp = evenNode;
        evenNode = evenNode->next;
        if(evenNode != NULL) {
          evenNode->prev = temp;
          oddNode = evenNode->next;
        }
      }
    }
  }   
}    

//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 five elements in the list.
  push_back(&MyList, 10);
  push_back(&MyList, 20);
  push_back(&MyList, 30);
  push_back(&MyList, 40);
  push_back(&MyList, 50);

  //Display the content of the list.
  PrintList(MyList);

  //delete odd nodes of the list
  deleteOddNodes(&MyList);

  printf("After deleting odd nodes.\n");
  //Display the content of the list.
  PrintList(MyList);

  return 0; 
}

The above code will give the following output:

The list contains: 10 20 30 40 50 
After deleting odd nodes.
The list contains: 20 40
# 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 odd nodes of the list
  def deleteOddNodes(self):  
    if (self.head != None):
      temp = self.head
      self.head = self.head.next
      temp = None
      if (self.head != None):
        self.head.prev = None
        evenNode = self.head
        oddNode = self.head.next 
        while(evenNode != None and oddNode != None):
          evenNode.next = oddNode.next
          oddNode = None
          temp = evenNode
          evenNode = evenNode.next
          if(evenNode != None):
            evenNode.prev = temp
            oddNode = evenNode.next

  #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 five elements in the list.
MyList.push_back(10)
MyList.push_back(20)
MyList.push_back(30)
MyList.push_back(40)
MyList.push_back(50)

#Display the content of the list.
MyList.PrintList()

#delete odd nodes of the list
MyList.deleteOddNodes()

print("After deleting odd nodes.")
#Display the content of the list.
MyList.PrintList()

The above code will give the following output:

The list contains: 10 20 30 40 50 
After deleting odd nodes.
The list contains: 20 40
//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 odd nodes of the list
  void deleteOddNodes() {
    if(this.head != null) {
      Node temp = this.head;
      this.head = this.head.next;
      temp = null;
      if(this.head != null) {
        this.head.prev = null;
        Node evenNode = this.head;
        Node oddNode = this.head.next; 
        while(evenNode != null && oddNode != null) {
          evenNode.next = oddNode.next;
          oddNode = null;
          temp = evenNode;
          evenNode = evenNode.next;
          if(evenNode != null) {
            evenNode.prev = temp;
            oddNode = evenNode.next;
          }
        }
      }
    }
  }   

  //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 five elements in the list.
    MyList.push_back(10);
    MyList.push_back(20);
    MyList.push_back(30);
    MyList.push_back(40);
    MyList.push_back(50);

    //Display the content of the list.
    MyList.PrintList();

    //delete odd nodes of the list
    MyList.deleteOddNodes();
 
    System.out.println("After deleting odd nodes.");
    //Display the content of the list.
    MyList.PrintList();
  }
}

The above code will give the following output:

The list contains: 10 20 30 40 50 
After deleting odd nodes.
The list contains: 20 40
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 odd nodes of the list
  public void deleteOddNodes() {
    if(this.head != null) {
      Node temp = this.head;
      this.head = this.head.next;
      temp = null;
      if(this.head != null) {
        this.head.prev = null;
        Node evenNode = this.head;
        Node oddNode = this.head.next; 
        while(evenNode != null && oddNode != null) {
          evenNode.next = oddNode.next;
          oddNode = null;
          temp = evenNode;
          evenNode = evenNode.next;
          if(evenNode != null) {
            evenNode.prev = temp;
            oddNode = evenNode.next;
          }
        }
      }
    }
  }  

  //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 five elements in the list.
    MyList.push_back(10);
    MyList.push_back(20);
    MyList.push_back(30);
    MyList.push_back(40);
    MyList.push_back(50);

    //Display the content of the list.
    MyList.PrintList();  

    //delete odd nodes of the list
    MyList.deleteOddNodes();

    Console.WriteLine("After deleting odd nodes.");
    //Display the content of the list.
    MyList.PrintList();
  }
}

The above code will give the following output:

The list contains: 10 20 30 40 50 
After deleting odd nodes.
The list contains: 20 40
<?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 odd nodes of the list
  public function deleteOddNodes() {
    if($this->head != null) {
      $temp = $this->head;
      $this->head = $this->head->next;
      $temp = null;
      if($this->head != null) { 
        $this->head->prev = null;
        $evenNode = $this->head;
        $oddNode = $this->head->next; 
        while($evenNode != null && $oddNode != null) {
          $evenNode->next = $oddNode->next;
          $oddNode = null;
          $temp = $evenNode;
          $evenNode = $evenNode->next;
          if($evenNode != null) {
            $evenNode->prev = $temp;
            $oddNode = $evenNode->next;
          }
        }
      }
    }
  }    

  //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 five elements in the list.
$MyList->push_back(10);
$MyList->push_back(20);
$MyList->push_back(30);
$MyList->push_back(40);
$MyList->push_back(50);

//Display the content of the list.
$MyList->PrintList();

//delete odd nodes of the list
$MyList->deleteOddNodes();

echo "After deleting odd nodes.\n";
//Display the content of the list.
$MyList->PrintList();
?>

The above code will give the following output:

The list contains: 10 20 30 40 50 
After deleting odd nodes.
The list contains: 20 40

5