reverse double linked list using recursion

Question

I am writing program to reverse double linked list using recursion. I know how to implement iteratively. but stuck in recursion.

Here is what I have

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

I see the prev pointers are not set properly. but next pointers are in fact correct.

Thanks

Answer 1

In general, one way to simplify thinking about recursive code is to only consider the guarantees that are offered when a recursive call is complete. You then need to check that these guarantees hold in the basic case that stops recursion (eg empty list, or list with a single node), and that they are maintained with each additional recursive call.

There are a few issues in the code provided:

• the condition that stopped recursion did not handle correctly the situations when node.next == null . The node.prev field should have been set to null in that case.

• the newNode variable name may cause some confusion. It represents the new head of the reversed list (which was the tail of the initial list). I renamed it to newHead .

• related to the point above, the assignment node.prev=newNode; is not correct -- we do not want the prev of each node to point to the head of the new list.

After incorporating these aspects, the code below correctly reverses the list:

public static Node reverseRecurseDoubleLinkedList(Node node) {
    if (node == null) {
        return node;
    }

    if (node.next == null) {
        node.prev = null;
        return node;
    }

    Node newHead = reverseRecurseDoubleLinkedList(node.next);
    node.next.next = node;
    node.prev = node.next;
    node.next = null;

    return newHead;
}