Given the `head` of a singly linked list, reverse the list, and return the reversed list.

Example 1:

    Input: head = [1,2,3,4,5]
    Output: [5,4,3,2,1]

Example 2:

    Input: head = [1,2]
    Output: [2,1]

Example 3:

    Input: head = []
    Output: []

Constraints:

- The number of nodes in the list is the range `[0, 5000]`.
- `-5000 <= Node.val <= 5000`

Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both?