Merge Two Sorted Linked Lists

Combine two sorted linked lists into one sorted list and return its head.

You get the heads of two singly linked lists, a and b. Each list is sorted in ascending order (smallest first). The result must also be in ascending order.

• Build the result by reusing the existing nodes. Relink their next references. Don't create new nodes.
• Either list can be null (empty). If both are null, return null.
• Values can repeat, inside one list and across both lists. Values can be negative.