Given the head of a singly linked list, reverse the list, and return the reversed list.
Input: head = [1,2,3,4,5]
Output: [5,4,3,2,1]
Input: head = [1,2]
Output: [2,1]
The number of nodes in the list is in the range [0, 5000].-5000 <= Node.val <= 5000