234. 回文链表

题目链接	234. 回文链表
思路	链表综合题：快慢指针 + 指针翻转
题解链接	官方题解
关键点	while fast.next and fast.next.next: ...
时间复杂度	\(O(n)\)
空间复杂度	\(O(1)\)

代码实现：

class Solution:
    def isPalindrome(self, head: Optional[ListNode]) -> bool:
        if head is None:
            return True
        first = self.halfList(head)
        second = self.reverseList(first.next)

        answer = True
        cur_first, cur_second = head, second
        while answer and cur_second:
            if cur_first.val != cur_second.val:
                answer = False
                break
            cur_first = cur_first.next
            cur_second = cur_second.next
        first.next = self.reverseList(second)
        return answer
    
    def halfList(self, head):
        slow = fast = head
        while fast.next and fast.next.next:
            fast = fast.next.next
            slow = slow.next
        return slow
    
    def reverseList(self, head):
        previous = None
        current = head
        while current:
            next_node = current.next
            current.next = previous
            previous = current
            current = next_node
        return previous