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Python - Delete all nodes by key of the Doubly Linked List



In this method, all nodes in the 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 is deleted, the list becomes 10->30->10.

If the head of the 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.

def pop_all(self, key):   
  
  #1. if the head is not null and value stored at head
  #   is equal to key, keep on adjusting the head as
  #   head next, delete previous head and adjust links
  #   until new head becomes null or not equal to key
  while(self.head != None and self.head.data == key):
    nodeToDelete = self.head
    self.head = self.head.next
    nodeToDelete = None
    if (self.head != None):
      self.head.prev = None

  #2. create a temp node to traverse through the
  #   list and delete nodes with value equal to 
  #   key and adjust links accordingly
  temp = self.head        
  if(temp != None):
    while(temp.next != None):
      if(temp.next.data == key):
        nodeToDelete = temp.next
        temp.next = temp.next.next
        if(temp.next != None):
          temp.next.prev = temp  
        nodeToDelete = None
      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 doubly linked list.

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

#class LinkedList
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 all nodes by key
  def pop_all(self, key):   
    while(self.head != None and self.head.data == key):
      nodeToDelete = self.head
      self.head = self.head.next
      nodeToDelete = None
      if (self.head != None):
        self.head.prev = None

    temp = self.head        
    if(temp != None):
      while(temp.next != None):
        if(temp.next.data == key):
          nodeToDelete = temp.next
          temp.next = temp.next.next
          if(temp.next != None):
            temp.next.prev = temp  
          nodeToDelete = None
        else:
          temp = temp.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 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()

The above code will give the following output:

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

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