给定一个单链表 L：L0→L1→…→Ln-1→Ln ， 将其重新排列后变为： L0→Ln→L1→Ln-1→L2→Ln-2→… 你不能只是单纯的改变节点内部的值，而是需要实际的进行节点交换。
示例 1: 给定链表 1->2->3->4, 重新排列为 1->4->2->3. 示例 2: 给定链表 1->2->3->4->5, 重新排列为 1->5->2->4->3.
class Solution:
def reorderList(self, head: ListNode) -> None:
if not head:
return
mid = self.middleNode(head)
l1 = head
l2 = mid.next
mid.next = None
l2 = self.reverseList(l2)
self.mergeList(l1, l2)
def middleNode(self, head: ListNode) -> ListNode:
slow = fast = head
while fast.next and fast.next.next:
slow = slow.next
fast = fast.next.next
return slow
def reverseList(self, head: ListNode) -> ListNode:
prev = None
curr = head
while curr:
nextTemp = curr.next
curr.next = prev
prev = curr
curr = nextTemp
return prev
def mergeList(self, l1: ListNode, l2: ListNode):
while l1 and l2:
l1_tmp = l1.next
l2_tmp = l2.next
l1.next = l2
l1 = l1_tmp
l2.next = l1
l2 = l2_tmp