AGAIN!?

public static DoublyLinkedListNode reverse(DoublyLinkedListNode llist)
{
    return llist;
}

Solution.cs(34,25): warning CS8625: Cannot convert null literal to non-nullable reference type. Solution.cs(35,25): warning CS8625: Cannot convert null literal to non-nullable reference type. 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. 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. Solution.cs(24,25): warning CS8625: Cannot convert null literal to non-nullable reference type. Solution.cs(25,25): warning CS8625: Cannot convert null literal to non-nullable reference type. 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. 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. Solution.cs(110,43): error CS0103: The name 'reverse' does not exist in the current context

These lines are not editable, so what the heck are you supposed to do?

Solution.cs(96,50): warning CS8604: Possible null reference argument for parameter 'path' in 'StreamWriter.StreamWriter(string path, bool append)'.

My answer in Python

def reverse(llist):
    prev = None
    current = llist
    while current:
        next_node = current.next
        current.next = prev
        current.prev = next_node
        prev = current
        current = next_node
    return prev

Java

Iterative

public static DoublyLinkedListNode reverse(DoublyLinkedListNode llist) {
	DoublyLinkedListNode newHead = null;
	while(llist != null){
		DoublyLinkedListNode next = llist.next;
		llist.next = newHead;
		llist.prev = next;
		newHead = llist;
		llist = next;
	}
	return newHead;
}

Recursive:

public static DoublyLinkedListNode reverse(DoublyLinkedListNode llist) {
	return helper(null, llist);
}

private static DoublyLinkedListNode helper(DoublyLinkedListNode prev, DoublyLinkedListNode node){
	if(node == null)
		return prev;
	DoublyLinkedListNode next = node.next;
	node.next = prev;
	node.prev = next;
	return helper(node, next);
}

This and "reverse a linked list" code is bugged from the beggining in Java 8 (the only Java that has autocomplete), is there any fu__ing checking in this website ?

Java 15 O(n)

public static DoublyLinkedListNode reverse(DoublyLinkedListNode llist) {
        DoublyLinkedListNode current = llist;
        DoublyLinkedListNode prev = null;

        while (current != null) {
            DoublyLinkedListNode nextNode = current.next;
            current.next = prev;
            current.prev = nextNode; // swap prev and next pointers
            prev = current;
            current = nextNode;
        }

        //prev points to the new head of the reversed list
        return prev;
    }