Circular Linked List Data Structure | DSA Tutorials

YASH PAL,

Circular Linked List – In Data Structure, a circular linked list is a list in which there is no None link. and the link of the last node refers to the first node. Each node has a successor, and all the nodes form a circle.

Circular Linked List Data Structure
Figure 1: Circular Linked List

To perform the operations on a circular linked list, we will generally need to store the reference to the last node of the list. because if we store the reference of the first node, then we can’t perform operations such as insertion, deletion, etc in constant time. And we need to traverse the whole linked list. So, to perform all the operations in constant time, we always store the reference to the last node.

Figure 2: Operations on Circular Linked List

We also use the circular linked list to implement other data structure like the implementation of a queue. because the insertions and deletions in the queue take constant time using a circular linked list.

Python program to implement a circular linked list

class Node(object):

    def __init__(self, value):
        self.info = value
        self.link = None

class CircularLinkedList(object):

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

        if self.last == None:
            print("List is empty\n")
            return        p = self.last.link

        while True:
            print(p.info, " ", end='')
            p = p.link
            if p == self.last.link:
                break        print()

    def insert_in_beginning(self, data):
        temp = Node(data)
        temp.link = self.last.link
        self.last.link = temp

    def insert_in_empty_list(self, data):
        temp = Node(data)
        self.last = temp
        self.last.link = self.last

    def insert_at_end(self,data):
        temp = Node(data)
        temp.link = self.last.link
        self.last.link = temp
        self.last = temp

    def create_list(self):
        n = int(input("Enter the number of nodes : "))
        if n == 0:
            return        data = int(input("Enter the element to be inserted : "))
        self.insert_in_empty_list(data)

        for i in range(n-1):
            data = int(input("Enter the element to be inserted : "))
            self.insert_at_end(data)

    def insert_after(self, data, x):
        p = self.last.link

        while True:
            if p.info == x:
                break            p = p.link
            if p == self.last.link:
                break
        if p == self.last.link and p.info != x:
            print(x, " not present in the list")
        else:
            temp = Node(data)
            temp.link = p.link
            p.link = temp
            if p == self.last:
                self.last = temp

    def delete_first_node(self):
        if self.last is None:
            return        if self.last.link == self.last:
            self.last = None            return        self.last.link = self.last.link.link

    def delete_last_node(self):
        if self.last is None:
            return        if self.last.link == self.last:
            self.last = None            return
        p = self.last.link
        while p.link != self.last:
            p = p.link
        p.link = self.last.link
        self.last = p

    def delete_node(self,x):

        if self.last is None:
            return        if self.last.link == self.last and self.last.info == x:
            self.last = None            return        if self.last.link.info == x:
            self.last.link == self.last.link.link
            return
        p = self.last.link
        while p.link != self.last.link:
            if p.link.info == x:
                break            p = p.link

        if p.link == self.last.link:
            print(x, " not found in the list")
        else:
            p.link = p.link.link
            if self.last.info == x:
                self.last = p

#################################################

list = CircularLinkedList()


list.create_list()

while True:
    print("1. Display list")
    print("2. Insert in empty list")
    print("3. Insert a node in the beginning of the list")
    print("4. Insert a node at the end of the list")
    print("5. Insert a node after a specified node")
    print("6. Delete first node")
    print("7. Delete last node")
    print("8. Delete any node")
    print("9. Quite")

    option = int(input("Enter your choice : "))

    if option == 1:
        list.display_list()
    elif option == 2:
        data = int(input("Enter the element to be inserted : "))
        list.insert_in_empty_list(data)
    elif option == 3:
        data = int(input("Enter the element to be inserted : "))
        list.insert_in_beginning(data)
    elif option == 4:
        data = int(input("Enter the element to be inserted : "))
        list.insert_at_end(data)
    elif option == 5:
        data = int(input("Enter the element to be inserted : "))
        x = int(input("Enter the element after which to insert : "))
        list.insert_after(data,x)
    elif option == 6:
        list.delete_first_node()
    elif option == 7:
        list.delete_last_node()
    elif option == 8:
        data = int(input("Enter the element to be deleted : "))
        list.delete_node(data)
    elif option == 9:
        break    else:
        print("Wrong option")
    print()
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