Reverse a linked list in java

If you want to practice data structure and algorithm programs, you can go through data structure and algorithm interview questions.

In this post, we will see how to reverse a linked list in java.

This is one of popular interview question.

Java Linked List Interview Programs:

• How to reverse a linked list in pairs
• How to find middle element of linked list in java
• How to detect a loop in linked list in java
• Find start node of loop in linkedlist
• How to find nth element from end of linked list
• How to check if linked list is palindrome in java
• Add two numbers represented by linked list in java

There can be two solution to reverse a linked list in java

• Iterative
• Recursive

Iterative

Logic for this would be:

public static Node reverseLinkedList(Node currentNode)
    {
        // For first node, previousNode will be null
        Node previousNode=null;
        Node nextNode;
        while(currentNode!=null)
        {
            nextNode=currentNode.next;
            // reversing the link
            currentNode.next=previousNode;
            // moving currentNode and previousNode by 1 node
            previousNode=currentNode;
            currentNode=nextNode;
        }
        return previousNode;
    }

Java program to reverse a linkedlist will be :

package org.arpit.java2blog;

public class LinkedList{

    private Node head;

    private static class Node {
        private int value;
        private Node next;

        Node(int value) {
            this.value = value;

        }
    }

    public void addToTheLast(Node node) {

        if (head == null) {
            head = node;
        } else {
            Node temp = head;
            while (temp.next != null)
                temp = temp.next;

            temp.next = node;
        }
    }

    public void printList(Node head) {
        Node temp = head;
        while (temp != null) {
            System.out.format("%d ", temp.value);
            temp = temp.next;
        }
        System.out.println();
    }

    // Reverse linkedlist using this function
    public static Node reverseLinkedList(Node currentNode)
    {
        // For first node, previousNode will be null
        Node previousNode=null;
        Node nextNode;
        while(currentNode!=null)
        {
            nextNode=currentNode.next;
            // reversing the link
            currentNode.next=previousNode;
            // moving currentNode and previousNode by 1 node
            previousNode=currentNode;
            currentNode=nextNode;
        }
        return previousNode;
    }

    public static void main(String[] args) {
        LinkedList list = new LinkedList();
        // Creating a linked list
        Node head=new Node(5);
        list.addToTheLast(head);
        list.addToTheLast(new Node(6));
        list.addToTheLast(new Node(7));
        list.addToTheLast(new Node(1));
        list.addToTheLast(new Node(2));

        list.printList(head);
        //Reversing LinkedList
        Node reverseHead=reverseLinkedList(head);
        System.out.println("After reversing");
        list.printList(reverseHead);

    }

}

Run above program, you will get following output:

5 6 7 1 2
After reversing
2 1 7 6 5

Recursive

public static Node reverseLinkedList(Node node) {
        if (node == null || node.next == null) {
            return node;
        }

        Node remaining = reverseLinkedList(node.next);
        node.next.next = node;
        node.next = null;
        return remaining;
    }

Output of this program will be same as above program.
Now lets understand logic for above recursive program.
5->6->7->1->2
Above function will terminate when last node(2)‘s next will be null.so while returning when you reach at node with value 1,If you closely observe node.next.next=node is actually setting 2->1(i.e. reversing the link between node with value 1 and 2) and node.next=nullis removing link 1->2. So in each iteration, you are reversing link between two nodes.
Below diagram will make it clear

Please go through java interview programs for more such programs.

That’s all about reverse a linked list in java.