2.1. Reverse a singly linked list.

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

public class LinkedList {
Node head;

public void reverse() {
Node prev = null;
Node curr = head;
Node next = null;
while (curr != null) {
next = curr.next;
curr.next = prev;
prev = curr;
curr = next;
}
head = prev;
}

public void printList() {
Node temp = head;
while (temp != null) {
System.out.print(temp.data + " ");
temp = temp.next;
}
}

public static void main(String[] args) {
LinkedList list = new LinkedList();
list.head = new Node(1);
list.head.next = new Node(2);
list.head.next.next = new Node(3);
list.head.next.next.next = new Node(4);

System.out.println("Original List:");
list.printList();

list.reverse();

System.out.println("\nReversed List:");
list.printList();
}
}