Given the head of a linked list, return the list after sorting it in ascending order.

  Example 1:

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

Example 2:

Input: head = [-1,5,3,4,0] Output: [-1,0,3,4,5]

Example 3:

Input: head = [] Output: []

  Constraints:

The number of nodes in the list is in the range [0, 5 * 10⁴].
-10⁵ <= Node.val <= 10⁵

  Follow up: Can you sort the linked list in O(n logn) time and O(1) memory (i.e. constant space)?