04-leetcode-回文链表

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def isPalindrome(self, head: ListNode) -> bool:
        """要完成回文链表，首先第一步找到中间点（采取快慢指针查找）
        第二步，将查找的中间点后的链表反转
        第三步，对比两个链表的值"""
        #快慢指针
        slow,fast = head,head
        #回文最起码的条件
        if not head or not head.next:
            return True
        #找到中间点
        while fast.next and fast.next.next:
            slow,fast = slow.next,fast.next.next
        cur = slow.next #另一半链表头起点
        last = None
        #反转链表
        while cur:
            before = cur.next #防止next断裂
            cur.next = last
            last = cur
            cur = before
        #对比两个链表的值
        while last:
            if last.val != head.val:
                return False
            last,head = last.next,head.next
        return True