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Problem Statement

You have given a singly linked list, write a program to reverse the linked list. Try to do it in-place by changing links between the nodes.

Example 

Input :- head = [5, 4, 10, 15, 1]
Output :- [1, 15, 10, 4, 5]

Input :- head = [5, 4, 10, 15, 1, 5, 6]
Output :- [6, 5, 1, 15, 10, 4, 5]

Solution

This problem can be solved in following steps :-

1. Create three variables previous, current, next. Initially assign previous to null, current to head node of linked list and next to second node of linked list.
2. Reassign next of current variable node to previous.
3. Move all variables by one node, i.e previous = current, current = next and next = next.next
4. Go to step 2 until the current variable is not null.
5. return previous variable.

Program

public class Main {

	public static void main(String[] args) {
		
		ListNode head = null;
		
		ListNode node = new ListNode(5);
		head = node;		
		node.next = new ListNode(4);
		node = node.next;		
		node.next = new ListNode(10);
		node = node.next;		
		node.next = new ListNode(15);
		node = node.next;		
		node.next = new ListNode(1);
		
		Main main = new Main();
		head = main.reverseList(head);
		
		ListNode listNode = head;
		while(listNode != null) {
			System.out.print(listNode.value + ", ");
			listNode = listNode.next;
		}
	}
	
	/* Solution */
	public ListNode reverseList(ListNode head) {        
        ListNode previous = null;
        ListNode current = head;
        ListNode next = null;        
        while(current != null){
            next = current.next;
            current.next = previous;
            previous = current;
            current = next;
        }
        return previous;
    }
	
	/* Definition of Linked List Node */
	public static class ListNode {		
		int value;
		ListNode next;
		
		public ListNode(int value) {
			this.value = value;
		}		
	}
}

Result

1, 15, 10, 4, 5,

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