LeetCode热题100-回文链表

_深海凉_2026-05-11 9:32

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 。

示例 1：
复制代码
输入：head = [1,2,2,1]
输出：true

思路一：记录所有值到列表中，然后判断这些值是否为回文数。时间复杂度On，空间复杂度On.

python 复制代码

class Solution:
    def isPalindrome(self, head: Optional[ListNode]) -> bool:
        arr = []
        cur = head
        while cur:
            arr.append(cur.val)
            cur = cur.next
        return arr == arr[::-1]

思路二：空间复杂度O1

快慢指针 找到链表中点
反转后半段链表
前后两段逐一比

python 复制代码

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def isPalindrome(self, head: Optional[ListNode]) -> bool:
        if not head or not head.next:
            return True
        # 先找到中点
        slow = fast = head
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
        
        def reverse(node):
            pre = None
            while node:
                tmp = node.next
                node.next = pre
                pre = node
                node = tmp
            return pre
        
        second = reverse(slow)
        # 前后半段逐一对比
        p1, p2 = head, second
        while p2:
            if p1.val != p2.val:
                return False
            p1 = p1.next
            p2 = p2.next
        return True