HTMLify

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:

    def get_second_last(self, head) -> Optional[ListNode]:
        if not head or not head.next:
            return
        while head.next.next:
            head = head.next
        return head

    def unlink_and_get_last(self, head) -> Optional[ListNode]:
        second_last = self.get_second_last(head)
        if not second_last:
            return
        last = second_last.next
        second_last.next = None
        return last

    def reorderList(self, head: Optional[ListNode]) -> None:
        """
        Do not return anything, modify head in-place instead.
        """
        current = head
        while current:
            last = self.unlink_and_get_last(current)
            if not last:
                break
            last.next = current.next
            current.next = last
            current = current.next
            current = current.next