# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reorderList(self, head: ListNode) -> None:
        """
        Do not return anything, modify head in-place instead.
        """
        if not head:
            return head
        mid = self.findMid(head)
        tail = self.reverseList(mid.next)
        mid.next = None
        dummy = ListNode()
        dummy.next = head
        cur = dummy
        while head and tail:
            cur.next = head
            cur = cur.next
            head = head.next
            cur.next = tail
            cur = cur.next
            tail = tail.next
        if head:
            cur.next = head
        return dummy.next
    def findMid(self, head):
        slow = fast = head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
        return slow
    def reverseList(self, head):
        pre, cur = None, head
        while cur:
            cur.next, pre, cur = pre, cur, cur.next
        return pre