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× Python Data Structures - Doubly Linked List Other Related Topics


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.

def pop_first(self, key):     
  
  temp = self.head
  #1. check if the head is not null
  if(temp != None):
    
    #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):
      nodeToDelete = self.head
      self.head = self.head.next
      nodeToDelete = None
      if (self.head != None):
        self.head.prev = None
    else:

      #3. Else, traverse to the node previous to the 
      #   node with value equal to key, and adjust links 
      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
          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.

# 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 first node by key
  def pop_first(self, key):        
    temp = self.head
    if(temp != None):
      if(temp.data == key):
        nodeToDelete = self.head
        self.head = self.head.next
        nodeToDelete = None
        if (self.head != None):
          self.head.prev = None
      else:
        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
            break
          temp = temp.next

  #display the content of the list
  def PrintList(self):
    temp = self.head
    if(temp != None):
      print("\nThe list contains:", end=" ")
      while (temp != None):
        print(temp.data, end=" ")
        temp = temp.next
    else:
      print("\nThe 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 the first occurrence of 20
MyList.pop_first(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 20