Merge Two Sorted Lists

You are given the heads of two sorted linked lists `list1` and `list2`. Merge the two lists into one **sorted** list. The list should be made by splicing together the nodes of the first two lists. Return _the head of the merged linked list_.   **Example 1:** **Input:** list1 = [1,2,4], list2 = [1,3,4] **Output:** [1,1,2,3,4,4] ``` **Example 2:** **Input:** list1 = [], list2 = [] **Output:** [] ``` **Example 3:** **Input:** list1 = [], list2 = [0] **Output:** [0] ```   **Constraints:** - The number of nodes in both lists is in the range `[0, 50]`. - `-100 <= Node.val <= 100` - Both `list1` and `list2` are sorted in **non-decreasing** order.