Doubly Linked List is an extension of Linked List in which we have two pointer variables, so we could traverse back and forth directions. It contains two pointers conventionally named, next and prev, in next we store the address or object of next element or node and in prev, we store the address of the previous object. Like a simple linked-list Doubly linked list also store its element sequentially so if we want to visit the last element, first we would have to traverse through all other nodes. Some Important terminology of Doubly Linked List: Node: A node is a collection of data value and two pointers next and prev Next: Next is a pointer which will hold the address or object of sequential next node. Prev: Prev is a pointer which holds the address or object of the previous node.
Doubly Linked List (DLL) Representation
- The first node of Doubly linked list is a head node and its prev pointer will hold Null Value
- Each node of the doubly Linked list will contain a data value and two pointers
- The next pointer will hold the address of the next node.
- The last node’s prev variable will contain a Null value.
DLL Basic Operations
Here are some operations which we can apply on a Doubly linked list:
- Insertion
- Deletion
- Insert Last
- Delete last
- Insert After
- Delete
- Forward Traversing
- Backward Traversing
Advantages and Disadvantages of DLL
Advantages
- With the Doubly Linked list, we can transverse back and forth directions.
- The Insertion and deletion operations on the doubly linked list are easier. Unlike simple linked list here we do not need to traverse to the predecessor node and store its reference. Rather, in a doubly-linked list, the reference of the predecessor node can be retrieved from the node that we want to delete.
Disadvantage
- Each node requires two variables which require more memory.
- We have to manage both the pointers to make an effective doubly-linked list data structure.
Doubly-Linked-List Implementation
Python
class Node:
def __init__(self, next=None, prev=None, data=None):
self.next = next
self.prev = prev
self.data = data
class DoublyLinkedList:
def __init__(self):
self.head = None
def Inset_at_Beginning(self, data):
new_node = Node(data = data)
new_node.next = self.head
new_node.prev = None
if self.head is not None:
self.head.prev = new_node
self.head = new_node
def Insert_at_Middle(self, prev, data):
if prev is None:
print("No such node")
return
new_node = Node(data = data)
new_node.next = prev.next
prev.next = new_node
new_node.prev = prev
if new_node.next is not None:
new_node.next.prev = new_node
def Insert_at_End(self, data):
new_node = Node(data = data)
last = self.head
new_node.next = None
if self.head is None:
new_node.prev = None
self.head = new_node
return
while (last.next is not None):
last = last.next
last.next = new_node
new_node.prev = last
def delete_list_element (self, data):
if self.head is None:
print("list has no element to delete")
return
if self.head.next is None:
if self.head.item == data:
self.head = None
else:
print("Item not found")
return
if self.head.data == data:
self.head = self.head.next
self.head.prev = None
return
n = self.head
while n.next is not None:
if n.data ==data:
break;
n = n.next
if n.next is not None:
n.prev.next = n.next
n.next.prev = n.prev
else:
if n.data == data:
n.prev.next = None
else:
print("Element not found")
def Traverse_complete_llist(self):
if self.head is None:
print("List has no element")
return
else:
n = self.head
while n is not None:
print(n.data , " ")
n = n.next
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