Python Program to Implement Doubly Linked List Operations

This is a Python program to create a doubly linked list and implement insertion, deletion and display operations on the list.

Problem Description

The program creates a doubly linked list and presents the user with a menu to perform various operations on the list.

Problem Solution

1. Create a class Node with instance variables data and next.
2. Create a class DoublyLinkedList with instance variables first and last.
3. The variable first points to the first element in the doubly linked list while last points to the last element.
4. Define methods get_node, insert_after, insert_before, insert_at_beg, insert_at_end, remove and display.
5. get_node takes an index as argument and traverses the list from the first node that many times to return the node at that index.
6. The methods insert_after and insert_before insert a node after or before some reference node in the list.
7. The methods insert_at_beg and insert_at_end insert a node at the first or last position of the list.
8. The method remove takes a node as argument and removes it from the list.
9. The method display traverses the list from the first node and prints the data of each node.
10. Create an instance of DoublyLinkedList and present the user with a menu to perform operations on the list.

Program/Source Code

Here is the source code of a Python program to create a doubly linked list and implement insertion, deletion and display operations on the list. The program output is shown below.

```class Node:
def __init__(self, data):
self.data = data
self.next = None
self.prev = None

def __init__(self):
self.first = None
self.last = None

def get_node(self, index):
current = self.first
for i in range(index):
if current is None:
return None
current = current.next
return current

def insert_after(self, ref_node, new_node):
new_node.prev = ref_node
if ref_node.next is None:
self.last = new_node
else:
new_node.next = ref_node.next
new_node.next.prev = new_node
ref_node.next = new_node

def insert_before(self, ref_node, new_node):
new_node.next = ref_node
if ref_node.prev is None:
self.first = new_node
else:
new_node.prev = ref_node.prev
new_node.prev.next = new_node
ref_node.prev = new_node

def insert_at_beg(self, new_node):
if self.first is None:
self.first = new_node
self.last = new_node
else:
self.insert_before(self.first, new_node)

def insert_at_end(self, new_node):
if self.last is None:
self.last = new_node
self.first = new_node
else:
self.insert_after(self.last, new_node)

def remove(self, node):
if node.prev is None:
self.first = node.next
else:
node.prev.next = node.next

if node.next is None:
self.last = node.prev
else:
node.next.prev = node.prev

def display(self):
current = self.first
while current:
print(current.data, end = ' ')
current = current.next

print('insert <data> after <index>')
print('insert <data> before <index>')
print('insert <data> at beg')
print('insert <data> at end')
print('remove <index>')
print('quit')

while True:
print('The list: ', end = '')
a_dllist.display()
print()
do = input('What would you like to do? ').split()

operation = do[0].strip().lower()

if operation == 'insert':
data = int(do[1])
position = do[3].strip().lower()
new_node = Node(data)
suboperation = do[2].strip().lower()
if suboperation == 'at':
if position == 'beg':
a_dllist.insert_at_beg(new_node)
elif position == 'end':
a_dllist.insert_at_end(new_node)
else:
index = int(position)
ref_node = a_dllist.get_node(index)
if ref_node is None:
print('No such index.')
continue
if suboperation == 'after':
a_dllist.insert_after(ref_node, new_node)
elif suboperation == 'before':
a_dllist.insert_before(ref_node, new_node)

elif operation == 'remove':
index = int(do[1])
node = a_dllist.get_node(index)
if node is None:
print('No such index.')
continue
a_dllist.remove(node)

elif operation == 'quit':
break```
Program Explanation

1. An instance of DoublyLinkedList is created.
2. The user is presented with a menu to perform various operations on the list.
3. The corresponding methods are called to perform each operation.

Runtime Test Cases
```Case 1:
insert <data> after <index>
insert <data> before <index>
insert <data> at beg
insert <data> at end
remove <index>
quit
The list:
What would you like to do? insert 5 at beg
The list: 5
What would you like to do? insert 3 at beg
The list: 3 5
What would you like to do? insert 1 at end
The list: 3 5 1
What would you like to do? insert 10 after 1
The list: 3 5 10 1
What would you like to do? insert 0 before 2
The list: 3 5 0 10 1
What would you like to do? remove 4
The list: 3 5 0 10
What would you like to do? remove 1
The list: 3 0 10
What would you like to do? remove 5
No such index.
The list: 3 0 10
What would you like to do? quit

Case 2:
insert <data> after <index>
insert <data> before <index>
insert <data> at beg
insert <data> at end
remove <index>
quit
The list:
What would you like to do? insert 3 after 0
No such index.
The list:
What would you like to do? insert 2 at beg
The list: 2
What would you like to do? insert 3 before 0
The list: 3 2
What would you like to do? remove 0
The list: 2
What would you like to do? remove 0
The list:
What would you like to do? quit```

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