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Java Program to insert a new node at the middle of the singly linked list
In the following example of Java program to insert a new node at the middle of a singly linked list:
Program :
import java.util.Scanner;

public class SinglyLinkedList {
    
    // Node class
    static class Node {
        int data;
        Node next;
        
        Node(int data) {
            this.data = data;
            this.next = null;
        }
    }
    
    // Head of the linked list
    Node head;
    
    // Method to add a node to the end of the list
    public void add(int data) {
        Node newNode = new Node(data);
        if (head == null) {
            head = newNode;
        } else {
            Node current = head;
            while (current.next != null) {
                current = current.next;
            }
            current.next = newNode;
        }
    }
    
    // Method to insert a node at the middle of the list
    public void insertMiddle(int data) {
        if (head == null) {
            head = new Node(data);
            return;
        }
        Node slow = head;
        Node fast = head.next;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        Node newNode = new Node(data);
        newNode.next = slow.next;
        slow.next = newNode;
    }
    
    // Method to display the list
    public void display() {
        Node current = head;
        while (current != null) {
            System.out.print(current.data + " ");
            current = current.next;
        }
    }
    
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        SinglyLinkedList list = new SinglyLinkedList();
        System.out.print("Enter the number of nodes: ");
        int n = input.nextInt();
        for (int i = 1; i <= n; i++) {
            System.out.print("Enter the data for node " + i + ": ");
            int data = input.nextInt();
            list.add(data);
        }
        System.out.println("The linked list is: ");
        list.display();
        System.out.print("\nEnter the data for the new node: ");
        int data = input.nextInt();
        list.insertMiddle(data);
        System.out.println("The linked list after inserting the new node is: ");
        list.display();
    }
    
}
Output :
Enter the number of nodes: 5
Enter the data for node 1: 8
Enter the data for node 2: 7
Enter the data for node 3: 2
Enter the data for node 4: 5
Enter the data for node 5: 13
The linked list is: 
8 7 2 5 13 
Enter the data for the new node: 3
The linked list after inserting the new node is: 
8 7 2 3 5 13
This program defines a Node class to represent a node in the linked list. Each node contains an integer data and a reference to the next node in the list. The SinglyLinkedList class contains a reference to the head of the list, methods to add nodes to the end of the list, insert a node at the middle of the list, and display the list.

In the add method, a new node is created with the specified data, and added to the end of the list. If the list is empty, the new node becomes the head. If the list is not empty, the method traverses the list until it reaches the last node, and adds the new node as its next node.

In the insertMiddle method, the method checks if the list is empty. If the list is empty, the method creates a new node with the specified data and sets it as the head of the list. If the list is not empty, the method uses two pointers, slow and fast, to traverse the list. slow moves one node at a time, while fast moves two nodes at a time. When fast reaches the end of the list, slow is at the middle of the list. The method then creates a new node with the specified data, and inserts it after slow.