Python Program to Implement Circular Doubly Linked List

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This is a Python program to implement a circular doubly linked list.

Problem Description

The program creates a circular 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 CircularDoublyLinkedList with instance variable first.
3. The variable first points to the first element in the circular doubly linked list.
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. It stops if it reaches the first node again.
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. insert_at_beg modifies the variable first to point to the new node.
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 until it reaches the first node again.
10. Create an instance of CircularDoublyLinkedList 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 implement a circular doubly linked list. The program output is shown below.

class Node:
    def __init__(self, data):
       self.data = data
       self.next = None
       self.prev = None
 
 
class CircularDoublyLinkedList:
    def __init__(self):
        self.first = None
 
    def get_node(self, index):
        current = self.first
        for i in range(index):
            current = current.next
            if current == self.first:
                return None
        return current
 
    def insert_after(self, ref_node, new_node):
        new_node.prev = ref_node
        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):
        self.insert_after(ref_node.prev, new_node)
 
    def insert_at_end(self, new_node):
        if self.first is None:
            self.first = new_node
            new_node.next = new_node
            new_node.prev = new_node
        else:
            self.insert_after(self.first.prev, new_node)
 
    def insert_at_beg(self, new_node):
        self.insert_at_end(new_node)
        self.first = new_node
 
    def remove(self, node):
        if self.first.next == self.first:
            self.first = None
        else:
            node.prev.next = node.next
            node.next.prev = node.prev
            if self.first == node:
                self.first = node.next
 
    def display(self):
        if self.first is None:
            return
        current = self.first
        while True:
            print(current.data, end = ' ')
            current = current.next
            if current == self.first:
                break
 
 
a_cdllist = CircularDoublyLinkedList()
 
print('Menu')
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_cdllist.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_cdllist.insert_at_beg(new_node)
            elif position == 'end':
                a_cdllist.insert_at_end(new_node)
        else:
            index = int(position)
            ref_node = a_cdllist.get_node(index)
            if ref_node is None:
                print('No such index.')
                continue
            if suboperation == 'after':
                a_cdllist.insert_after(ref_node, new_node)
            elif suboperation == 'before':
                a_cdllist.insert_before(ref_node, new_node)
 
    elif operation == 'remove':
        index = int(do[1])
        node = a_cdllist.get_node(index)
        if node is None:
            print('No such index.')
            continue
        a_cdllist.remove(node)
 
    elif operation == 'quit':
        break
Program Explanation

1. An instance of CircularDoublyLinkedList 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.

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Runtime Test Cases
Case 1:
Menu
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 at beg
The list: 3 
What would you like to do? insert 5 at end
The list: 3 5 
What would you like to do? insert 1 after 0
The list: 3 1 5 
What would you like to do? insert 2 after 2
The list: 3 1 5 2 
What would you like to do? remove 0
The list: 1 5 2 
What would you like to do? remove 2
The list: 1 5 
What would you like to do? remove 1
The list: 1 
What would you like to do? remove 0
The list: 
What would you like to do? quit
 
Case 2:
Menu
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 10 at end
The list: 10 
What would you like to do? insert 1 at beg
The list: 1 10 
What would you like to do? insert 5 before 0
The list: 1 10 5 
What would you like to do? insert 9 at beg
The list: 9 1 10 5 
What would you like to do? remove 3
The list: 9 1 10 
What would you like to do? quit

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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