题目链接：https://leetcode.cn/problems/palindrome-linked-list/
难度：简单

描述：
给你一个单链表的头节点 head ，请你判断该链表是否为回文链表。如果是，返回 true ；否则，返回 false 。

题解

  1. # Definition for singly-linked list.
  2. # class ListNode:
  3. #     def __init__(self, val=0, next=None):
  4. #         self.val = val
  5. #         self.next = next
  6. class Solution:
  7.     def isPalindrome(self, head: ListNode) -> bool:
  8.         def reverse(head):
  9.             if head is None or head.next is None:
  10.                 return head
  12.             pre = None
  13.             cur = head
  15.             while cur:
  16.                 temp = cur.next
  17.                 cur.next = pre
  18.                 pre = cur
  19.                 cur = temp
  21.             return pre
  23.         def get_mid(head):
  24.             slow, fast = head, head.next
  25.             while fast and fast.next:
  26.                 slow = slow.next
  27.                 fast = fast.next.next
  29.             return slow
  31.         mid_node = get_mid(head)
  32.         mid_node.next = reverse(mid_node.next)
  34.         left, right = head, mid_node.next
  35.         while right:
  36.             if left.val != right.val:
  37.                 mid_node.next = reverse(mid_node.next)
  38.                 return False
  39.             left = left.next
  40.             right = right.next
  42.         mid_node.next = reverse(mid_node.next)
  43.         return True