Insertion at the beginning in circular linked list

Given a Circular Linked List and an integer key representing a node. Insert the given key node at the beginning of the circular linked list.

Input: key = 5

Output: 5 -> 1 -> 7 -> 8 -> 10
Explanation: After inserting 5 at the beginning of the given circular linked list, it has elements as 5, 1, 7, 8, 10.

Input: key = 1

Output: 1 -> 2 -> 5 -> 7 -> 8 -> 10
Explanation: After inserting 1 at the beginning of the given circular linked list, it has elements as 1, 2, 5, 7, 8, 10.

Insertion at the beginning in circular linked list

To insert a new node at the beginning of a circular linked list, we first create the new node and allocate memory for it. If the list is empty (indicated by the last pointer being NULL), we make the new node point to itself. If the list already contains nodes then we set the new node’s next pointer to point to the current head of the list (which is last->next), and then update the last node’s next pointer to point to the new node. This maintains the circular structure of the list.

Step-by-step approach:

• Create a new node with the given value.
• Check Empty List (last == nullptr):
  • Make newNode->next point to itself.
• Insert at Beginning:
  • Set newNode->next to last->next.
  • Update last->next to newNode.

Below is the implementation of the above approach:

#include <iostream>
using namespace std;

class Node {
public:
    int data;
    Node* next;

    Node(int x) {
        data = x;
        next = nullptr;
    }
};

Node* insertAtBeginning(Node* last, int key) {
    Node* newNode = new Node(key);

    if (last == nullptr) {
        newNode->next = newNode;
        return newNode;
    }

    newNode->next = last->next;
    last->next = newNode;

    return last;
}

void printList(Node* last) {
    if (last == nullptr) return;

    Node* head = last->next;
    Node* temp = head;

    do {
        cout << temp->data;
        temp = temp->next;
        if (temp != head) cout << " -> ";
    } while (temp != head);

    cout << endl;
}

int main() {
    // Create circular linked list: 2 -> 3 -> 4
    Node* first = new Node(2);
    first->next = new Node(3);
    first->next->next = new Node(4);

    Node* last = first->next->next;
    last->next = first;

    // Insert 5 at the beginning
    last = insertAtBeginning(last, 5);

    printList(last);

    return 0;
}

class Node {
    int data;
    Node next;

    Node(int x) {
        data = x;
        next = null;
    }
}

class GfG {

    static Node insertAtBeginning(Node last, int key) {
        Node newNode = new Node(key);

        if (last == null) {
            newNode.next = newNode;
            return newNode;
        }

        newNode.next = last.next;
        last.next = newNode;

        return last;
    }

    static void printList(Node last) {
        if (last == null) return;

        Node head = last.next;
        Node temp = head;

        do {
            System.out.print(temp.data);
            temp = temp.next;
            if (temp != head) System.out.print(" -> ");
        } while (temp != head);

        System.out.println();
    }

    public static void main(String[] args) {
        // Create circular linked list: 2 -> 3 -> 4
        Node first = new Node(2);
        first.next = new Node(3);
        first.next.next = new Node(4);

        Node last = first.next.next;
        last.next = first;

        // Insert 5 at the beginning
        last = insertAtBeginning(last, 5);

        printList(last);
    }
}

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


def insertAtBeginning(last, key):
    newNode = Node(key)

    if last is None:
        newNode.next = newNode
        return newNode

    newNode.next = last.next
    last.next = newNode

    return last


def printList(last):
    if last is None:
        return

    head = last.next
    temp = head

    while True:
        print(temp.data, end="")
        temp = temp.next
        if temp != head:
            print(" -> ", end="")
        else:
            break
    print()


if __name__ == "__main__":
    # Create circular linked list: 2 -> 3 -> 4
    first = Node(2)
    first.next = Node(3)
    first.next.next = Node(4)

    last = first.next.next
    last.next = first

    # Insert 5 at the beginning
    last = insertAtBeginning(last, 5)

    printList(last)

using System;

class Node {
    public int data;
    public Node next;

    public Node(int x) {
        data = x;
        next = null;
    }
}

class GfG {
    public static Node insertAtBeginning(Node last, int key) {
        Node newNode = new Node(key);

        if (last == null) {
            newNode.next = newNode;
            return newNode;
        }

        newNode.next = last.next;
        last.next = newNode;

        return last;
    }

    public static void printList(Node last) {
        if (last == null) return;

        Node head = last.next;
        Node temp = head;

        while (true) {
            Console.Write(temp.data);
            temp = temp.next;
            if (temp != head) {
                Console.Write(" -> ");
            } else {
                break;
            }
        }
        Console.WriteLine();
    }

    static void Main(string[] args) {
        // Create circular linked list: 2 -> 3 -> 4
        Node first = new Node(2);
        first.next = new Node(3);
        first.next.next = new Node(4);

        Node last = first.next.next;
        last.next = first;

        // Insert 5 at the beginning
        last = insertAtBeginning(last, 5);

        printList(last);
    }
}

class Node {
    constructor(x) {
        this.data = x;
        this.next = null;
    }
}

// Function to insert at beginning of circular linked list
function insertAtBeginning(last, key) {
    let newNode = new Node(key);

    if (last === null) {
        newNode.next = newNode;
        return newNode;
    }

    newNode.next = last.next;
    last.next = newNode;

    return last;
}

// Function to print circular linked list
function printList(last) {
    if (last === null) return;

    let head = last.next;
    let temp = head;

    while (true) {
        process.stdout.write(temp.data.toString());
        temp = temp.next;
        if (temp !== head) {
            process.stdout.write(" -> ");
        } else {
            break;
        }
    }
    console.log();
}

// Driver code
let first = new Node(2);
first.next = new Node(3);
first.next.next = new Node(4);

let last = first.next.next;
last.next = first;

// Insert 5 at beginning
last = insertAtBeginning(last, 5);

printList(last);

5 -> 2 -> 3 -> 4

Time Complexity: O(1)
Auxiliary Space: O(1)