Reverse Double Linked List in C#

This is a question that often comes up in an interview.

The solution is simple, but under pressure it’s easy to get confused.

To begin with, initialise three node references:

current points to the head node
previous points to null
next points to null

Remember to point the tail to the head before we begin:

tail = head;

Now, iterate over the nodes using current, until there are no more:

while (current is not null)

Within the loop, first initialise next to look ahead:

next = current.next;

Now set current.next to previous:

current.next = previous;

Set current.previous to next:

current.previous = next;

Advance previous to current:

previous = current;

Lastly, advance current to next:

current = next;

That’s the loop finished. Outside the loop remember to set the head:

head = previous;

Here is the code, starting with the main method:

This is the output:

Iterating list from head to tail (normal)
        [1, 2, 3, 4, 5]

Iterating list from tail to head (reverse)
        [5, 4, 3, 2, 1]

Reversing List

Iterating list from head to tail (normal)
        [5, 4, 3, 2, 1]

Iterating list from tail to head (reverse)
        [1, 2, 3, 4, 5]

The very simple Node Class: