# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def reorderList(self , head ):
        # write code here
        if not head or not head.next: return
        
        # 快慢指標尋找中間節點
        fast = head
        slow = head
        while
 
 fast.next and fast.next.next:
            fast = fast.next.next
            slow = slow.next
            
        # 拆分連結串列，並將後半部分連結串列重新反轉
        after = slow.next
        slow.next = None
        pre = None
        while after:
            temp = after.next
            after.next = pre
            pre =
 
 after
            after = temp
            
        # 交叉合併合併
        first = head
        after = pre
        while first and after:
            ftemp = first.next
            atemp = after.next
            first.next = after
            first = ftemp
            after.next = first
            after =
 
 atemp