Understanding the reversal pattern

Many doubly linked list problems require us to reverse the entire list or a part of it. For some problems, we may have to perform a reversal many times along with other more complex operations. While we can reverse the list using loops in multiple passes, it is not the best way to do it, as the code is complicated and error-prone. Just like the singly linked list, the most concise and efficient way to reverse a doubly linked list is to use a single-pass in-place reversal algorithm, which is a very simple four-line algorithm.