My code in c++:

DoublyLinkedListNode* reverse(DoublyLinkedListNode* llist) {
    if(llist == NULL) return NULL;
    else if(llist->next == NULL) return llist;
   ``
    DoublyLinkedListNode* re = new DoublyLinkedListNode(llist->data);
    llist = llist->next;
    while(llist != NULL){
        DoublyLinkedListNode* x = new DoublyLinkedListNode(llist->data);
        x->next = re;
        re->prev = x;
        re = x;

    llist = llist->next;
}
return re;

}

This is the easiest and most efficient way I've found to reverse a doubly linked list in python 3

#!/bin/python3

import math
import os
import random
import re
import sys

class DoublyLinkedListNode:
    def __init__(self, node_data):
        self.data = node_data
        self.next = None
        self.prev = None

class DoublyLinkedList:
    def __init__(self):
        self.head = None
        self.tail = None

    def insert_node(self, node_data):
        node = DoublyLinkedListNode(node_data)

        if not self.head:
            self.head = node
        else:
            self.tail.next = node
            node.prev = self.tail


        self.tail = node

def print_doubly_linked_list(node, sep, fptr):
    while node:
        fptr.write(str(node.data))

        node = node.next

        if node:
            fptr.write(sep)

#
# Complete the 'reverse' function below.
#
# The function is expected to return an INTEGER_DOUBLY_LINKED_LIST.
# The function accepts INTEGER_DOUBLY_LINKED_LIST llist as parameter.
#

#
# For your reference:
#
# DoublyLinkedListNode:
#     int data
#     DoublyLinkedListNode next
#     DoublyLinkedListNode prev
#
#

def reverse(head):
    # Write your code here
    if head is None:
        return None
    curr = head
    while curr:
        curr.prev, curr.next = curr.next, curr.prev
        head = curr
        curr = curr.prev
    return head

if __name__ == '__main__':
    fptr = open(os.environ['OUTPUT_PATH'], 'w')

    t = int(input())

    for t_itr in range(t):
        llist_count = int(input())

        llist = DoublyLinkedList()

        for _ in range(llist_count):
            llist_item = int(input())
            llist.insert_node(llist_item)

        llist1 = reverse(llist.head)

        print_doubly_linked_list(llist1, ' ', fptr)
        fptr.write('\n')

    fptr.close()

This problem's C# definition of DoublyLinkedListNode and DoublyLinkedList are buggy.

The way the author of this problem defined these classes in C# requires that the classes' fields (i.e., prev, next, head, tail) be set to a non-null values before exiting the constructor, and the classes' constructors aren't written to allow these parameters to be passed in.

So it will always fail if you try to solve this problem in C#.

Here's the full exception:

/tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.cs(24,25): warning CS8625: Cannot convert null literal to non-nullable reference type. [/tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.csproj] /tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.cs(25,25): warning CS8625: Cannot convert null literal to non-nullable reference type. [/tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.csproj] /tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.cs(22,16): warning CS8618: Non-nullable field 'next' must contain a non-null value when exiting constructor. Consider declaring the field as nullable. [/tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.csproj] /tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.cs(22,16): warning CS8618: Non-nullable field 'prev' must contain a non-null value when exiting constructor. Consider declaring the field as nullable. [/tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.csproj] /tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.cs(34,25): warning CS8625: Cannot convert null literal to non-nullable reference type. [/tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.csproj] /tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.cs(35,25): warning CS8625: Cannot convert null literal to non-nullable reference type. [/tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.csproj] /tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.cs(33,16): warning CS8618: Non-nullable field 'head' must contain a non-null value when exiting constructor. Consider declaring the field as nullable. [/tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.csproj] /tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.cs(33,16): warning CS8618: Non-nullable field 'tail' must contain a non-null value when exiting constructor. Consider declaring the field as nullable. [/tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.csproj] /tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.cs(119,43): error CS0103: The name 'reverse' does not exist in the current context [/tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.csproj] /tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.cs(105,50): warning CS8604: Possible null reference argument for parameter 'path' in 'StreamWriter.StreamWriter(string path, bool append)'. [/tmp/submission/20241104/05/55/hackerrank-71c387b303654a6e1a4f4f7510a502c7/code/Solution.csproj]

DoublyLinkedListNode* reverse(DoublyLinkedListNode* llist) { DoublyLinkedListNode *curr; curr=llist; while(curr->next!=NULL){ curr=curr->next; } DoublyLinkedListNode *head; head=curr; DoublyLinkedListNode *temp; DoublyLinkedListNode *count=NULL; while(curr!=NULL){ temp=curr->prev; curr->next=temp; curr->prev=count; count=curr; curr=temp;

}hhere is three poimter approach

return head; }