Merge two sorted linked lists into one sorted linked list

Merging two sorted linked lists involves combining all nodes present in each of the two input linked lists into a single linked list while maintaining their sorted order.

To accomplish this, we need consume the smallest head node among both the linked lists and add it to the final list. We continue the process until all nodes in both the linked lists are added to the merged sorted linked list. This approach ensures that the new merged list preserves the sorted order while re-using all nodes from the original two lists.

With each iteration in the loop, we move one head node from l1 or l2 to the merged list. When one of l1 or l2 becomes empty, we just add the remainder to the merged list directly.