class Solution:
    def reorderList(self, head: ListNode) -> None:
        if not head or not head.next:
            return

        # Step 1: Find the Middle
        slow, fast = head, head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
        
        # 'slow' is now at the end of the first half.
        # We split the list here.
        second = slow.next
        slow.next = None # Cut the first half

        # Step 2: Reverse Second Half
        prev = None
        curr = second
        while curr:
            next_temp = curr.next
            curr.next = prev
            prev = curr
            curr = next_temp
        
        # 'prev' is now the head of the reversed second half
        
        # Step 3: Merge the Two Halves
        first, second = head, prev
        
        while second:
            # Save next pointers
            tmp1, tmp2 = first.next, second.next
            
            # Link first to second
            first.next = second
            # Link second to old first.next
            second.next = tmp1
            
            # Move pointers forward
            first, second = tmp1, tmp2