## 06 March 2010

### Reverse a Singly Linked List Recursively in Java

We have already seen how to reverse a singly linked list with illustrative pictures. Now lets see how we can do it recursively. In the previous problem we did it iteratively, now we shall do it recursively.
To attack any problem in a recursive approach, we need to be very clear about the end/boundary conditions. For a linked list, reverse of a null list or reverse of list of size 1 is going to be the same.
Reverse of a linked list of size x will be the reverse of the 'next' element followed by first.
A picture means a thousand words. So, here is what happens internally.

Now for the comprehensive Java code (reference for SinglyLinkedList implementation can be found here)

Cheers,

zsljulius said...

Valuable! Concise and great graph illustration! Thanks for your good work!

Good to see that it helped you!!

Unknown said...

Very Nice Blog.

Reverse can be done this way too..

node reverse(node n)
{

if(n==null || n.next == null)
return n;

reverse(n.next);
n.next.next = n;
n.next = null;

return n;
}

BragBoy said...

@gen: Thanks for your feedback! Your code is more simpler and neater..

Varun Pathak said...

@gen
your code although is reversing the list, its returning the tail pointer of the reversed list. Their is no way to access its head.

Neco said...

Hi, consider this:

public class Lists {

public static void main(String[] args) {

Node nodeE = new Node("e", null);
Node nodeD = new Node("d", nodeE);
Node nodeC = new Node("c", nodeD);
Node nodeB = new Node("b", nodeC);
Node nodeA = new Node("a", nodeB);

Node node = reverse(nodeA);

while (node != null) {
System.out.println(node.getValue());
node = node.getNext();
}
}

public static Node reverse(Node head) {

}

private static Node recursiveHelper(Node prev, Node curr) {

if (curr == null) {

return prev;

} else {

Node next = curr.getNext();
curr.setNext(prev);

return recursiveHelper(curr, next);
}
}
}

whizz.comp said...

@Varun -
Good catch dude - i have the same opinion as you - but here's a fix.

{
if (nd == null || nd.Link == null) return null;

return retNode == null ? nextNode : retNode;
}

Anonymous said...

package com.list;

public void insert(int data)
{
Node vNode = new Node();
vNode.setData(data);
}

public void display()
{
while(vCurr != null )
{
System.out.print(vCurr.getData()+" ");
vCurr = vCurr.getNext() ;
}
System.out.println();
}

public void reverseList(Node vCurr)
{
if(vCurr==null)
return;

Node vPrev = vCurr ;
vCurr = vCurr.getNext();

if(vCurr == null)
return ;

if(vCurr.getNext() == null)
{
}

reverseList(vCurr);

vCurr.setNext(vPrev);
vPrev.setNext(null);

}

public static void main(String args[])
{
for(int i=0;i<10;i++)
{
impl.insert(i);
}
impl.display();
System.out.println("After Reverse the list is as follows ");
impl.display();
}

}

package com.list;

public class Node {

private Integer data ;
private Node next;

public Integer getData() {
return data;
}
public void setData(Integer data) {
this.data = data;
}
public Node getNext() {
return next;
}
public void setNext(Node next) {
this.next = next;
}

}