You're given the heads of two sorted linked lists list1 and list2. Merge them into one sorted list by splicing the existing nodes together, and return its head.
This is the linked-list version of the merge step in Merge sort.
Example
Input: list1 = [1, 2, 4], list2 = [1, 3, 4]
Output: [1, 1, 2, 3, 4, 4]