Design a function to sort a linked list in O(n log n) time using the merge sort algorithm. The function should return the head of the sorted linked list. 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 Constraints: - The number of nodes in the linked list is in the range [0, 5 * 10^4]. - -10^5 <= Node.val <= 10^5.

``` class ListNode: def __init__(self, val=0, next=None): self.val = val self.next = next def sort_list(head): if not head or not head.next: return head def split(head): slow = fast = head while fast and fast.next: slow = slow.next fast = fast.next.next return head, slow left, right = split(head) left = sort_list(left) right = sort_list(right) return merge(left, right) def merge(left, right): if not left: return right if not right: return left if left.val < right.val: left.next = merge(left.next, right) return left else: right.next = merge(left, right.next) return right ```