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

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

If the head of the circular 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 and deleting previous head # until new head becomes null or not equal to key while(self.head != None and self.head.data == key): if(self.head.next == self.head): self.head = None else: nodeToDelete = self.head temp = self.head # traverse to the last element of # the list to adjust links while(temp.next != self.head): temp = temp.next self.head = self.head.next temp.next = self.head self.head.prev = temp nodeToDelete = 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 != self.head): if(temp.next.data == key): nodeToDelete = temp.next temp.next = temp.next.next 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 circular doubly linked list.

# 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 newNode.next = self.head newNode.prev = self.head return else: temp = self.head while(temp.next != self.head): temp = temp.next temp.next = newNode newNode.next = self.head newNode.prev = temp self.head.prev = newNode #Delete all nodes by key def pop_all(self, key): while(self.head != None and self.head.data == key): if(self.head.next == self.head): self.head = None else: nodeToDelete = self.head temp = self.head while(temp.next != self.head): temp = temp.next self.head = self.head.next temp.next = self.head self.head.prev = temp nodeToDelete = None temp = self.head if(temp != None): while(temp.next != self.head): if(temp.next.data == key): nodeToDelete = temp.next temp.next = temp.next.next 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 (True): print(temp.data, end=" ") temp = temp.next if(temp == self.head): break 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