143. 重排链表

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        if not head :
            return head
        
        pre =None
        cur =head 
        while cur :
            temp =cur.next 
            cur.next=pre
            pre =cur 
            cur =temp
        return pre 

    def reorderList(self, head: Optional[ListNode]) -> None:
        """
        Do not return anything, modify head in-place instead.
        """
        slow =head 
        fast =head.next 
        while  fast and fast.next :
            slow =slow.next 
            fast = fast.next.next 
        second = self.reverseList(slow.next)
        slow.next =None
        first =head 
        while  second :
             first_next= first.next 
             second_next =second.next 
             first.next =second
             second.next=first_next
             first=first_next
             second=second_next