LinkedList.java

import java.util.*;

public class LinkedList<T> {
	private Node<T> head = null;

	// LinkedList stucture
	private static class Node<T> {
		T data;
		Node<T> next;

		Node(T data) {
			this.data = data;
			next = null;
		}
	}

	// Add data last in linked list.
	public void addLast(T data) {
		Node<T> newNode = new Node<T>(data);
		if (head == null) {
			head = newNode;
		} else {
			Node<T> q = head;
			while (q.next != null)
				q = q.next;
			q.next = newNode;
		}
		System.out.println("Data added successfully.");
	}

	// Add data first in linked list.
	public void addFirst(T data) {
		Node<T> newNode = new Node<T>(data);
		if (head == null) {
			head = newNode;
		} else {
			newNode.next = head;
			head = newNode;
		}
		System.out.println("Data added successfully.");
	}

	// Change value at specific index.
	public void changeByIndex(int index, T data) {
		if (head == null) {
			System.out.println("Linked list is empty.");
		} else {
			Node<T> q = head;
			int i;
			for (i = 0; q != null && i < index; i++) {
				q = q.next;
			}
			if (q == null)
				System.out.println("Not available.");
			else {
				q.data = data;
				System.out.println("Data changed successfully.");
			}
		}
	}

	// Change By Data
	public void changeByData(T oldData, T newData) {
		if (head == null) {
			System.out.println("Linked list is empty.");
		} else {
			Node<T> q = head;
			while (q != null) {
				if (q.data.equals(oldData))
					q.data = newData;
				q = q.next;
			}
			System.out.println(oldData + " data changed to " + newData + " successfully.");
		}
	}

	// Delete at specific index.
	public void delete(int index) {
		if (head == null) {
			System.out.println("Linked list is empty.");
		} else {
			Node<T> q = head, pre = head;
			int i;

			for (i = 1; q != null && i < index; i++) {
				pre = q;
				q = q.next;
			}
			if (q == null || i < index)
				System.out.println("Not available.");
			else {
				System.out.println(q.data + " deleted successfully.");
				if (index == 0)
					head = head.next;
				else
					pre.next = q.next;
			}
		}
	}

	// Show the linked list.
	@Override
	public String toString() {
		if (head == null) {
			return "Linked list is empty.";
		}
		
		Node<T> q = head;
		String list="";
		while (q != null) {
			list += q.data + " -> ";
			q = q.next;
		}
		list += "null";
		return list;
	}

	public void reverse() {
		if (head == null) {
			System.out.println("Linked list is empty.");
		} else {
			Node<T> curr = head, next = null, pre = null;
			while (curr != null) {
				next = curr.next;
				curr.next = pre;
				pre = curr;
				curr = next;
			}
			head = pre;
			System.out.println("LinkedList is reversed.");
		}
	}

	public static void main(String arg[]) {
		LinkedList<String> ll = new LinkedList<>();
		Scanner sc = new Scanner(System.in);

		while (true) {
			System.out.println("1. Add the data at first");
			System.out.println("2. Add the data at last");
			System.out.println("3. Change by index");
			System.out.println("4. Change by data");
			System.out.println("5. Delete data at specific index");
			System.out.println("6. Reverse the LL.");
			System.out.println("7. Display LL");
			System.out.println("8. Exit");
			System.out.print("\nEnter your choice : ");
			int choice = sc.nextInt();
			switch (choice) {
				case 1:
					System.out.print("\nData : ");
					ll.addFirst(sc.next());
					break;
				case 2:
					System.out.print("\nData : ");
					ll.addLast(sc.next());
					break;
				case 3:
					System.out.print("\nIndex : ");
					int index = sc.nextInt();
					System.out.print("New Data : ");
					String data = sc.next();
					ll.changeByIndex(index, data);
					break;
				case 4:
					System.out.print("\nOld Data : ");
					String oldData = sc.next();
					System.out.print("New Data : ");
					String newData = sc.next();
					ll.changeByData(oldData, newData);
					break;
				case 5:
					System.out.print("\nIndex : ");
					ll.delete(sc.nextInt());
					break;
				case 6:
					ll.reverse();
					break;
				case 7:
					System.out.println(ll);
					break;
				case 8:
					sc.close();
					System.exit(0);
					break;
				default:
					System.out.println("You enter wrong choice.");
			}
			System.out.println();
		}
	}
}