Java Function Code to create Reverse Linked List of Singly Linked List (In memory pointer changes)

Here is a sample java Function Code to create Reverse Linked List of Singly Linked List (In memory pointer changes). Approach

• 1) Maintain a previous node pointer
• 2) maintain a current node pointer
• 3) store current-> next in temporary variable
• 4) replace the current node-> next to previous pointer
• 5) do step 3 and 4 till you reach current node is your end node
• 6) at the end update the head->next to null as the old linked list head will be your end node now
• 7) return previous node as this will be the head of your new linked list.

You can see the image below - before and after structure of linked list:

package com.sample.linkedlist;

public class ReverseLinkedList {

 static class Node {
  int data;
  Node next;

  public Node(int data) {
   this.data = data;
  }

  @Override
  public String toString() {
   return "Node[data=" + data + "]";
  }

 }

 public static Node reverse(Node head) {

  if (head == null)
   return null;

  Node currentNode = head.next;
  Node previousNode = head;
  while (currentNode != null) {
   System.out.println("previous:" + previousNode +" current:"+currentNode);
   
   Node tempCurrent = currentNode.next;

   currentNode.next = previousNode;
   previousNode = currentNode;

   /* move to next node */
   currentNode = tempCurrent;
  }
  head.next = null;

  return previousNode;
 }

 static void print(Node head) {
  while (head != null) {
   System.out.print(head + " ");
   head = head.next;
  }
  System.out.println();

 }

 public static void main(String[] args) {
  
  Node head = new Node(1);
  head.next = new Node(2);
  head.next.next = new Node(3);
  head.next.next.next=new Node(4);
  
  /*print actual linked list*/
  print(head);

  Node newHead = reverse(head);

  /*print reversed linked list*/
  print(newHead);
 }
}

Output

Node[data=1] Node[data=2] Node[data=3] Node[data=4] 
previous:Node[data=1] current:Node[data=2]
previous:Node[data=2] current:Node[data=3]
previous:Node[data=3] current:Node[data=4]
Node[data=4] Node[data=3] Node[data=2] Node[data=1] 

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