234-palindrome-linked-list.md

234. Palindrome Linked List - 回文链表

请判断一个链表是否为回文链表。

示例 1:

输入: 1->2
输出: false

示例 2:

输入: 1->2->2->1
输出: true

进阶：
你能否用 O(n) 时间复杂度和 O(1) 空间复杂度解决此题？

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题目标签：Linked List / Two Pointers

题目链接：LeetCode / LeetCode中国

题解

Language	Runtime	Memory
cpp	8 ms	4.5 MB

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool isPalindrome(ListNode* head) {
        if (head == NULL || head->next == NULL) {
            return true;
        }
        ListNode* dummy = new ListNode(0);
        dummy->next = head;
        ListNode* p = dummy;
        ListNode* q = head;
        while (q && q->next) {
            p = p->next;
            q = q->next->next;
        }

        ListNode* r = p->next;
        ListNode* s = r->next;
        if (s == NULL) {
            return p->val == r->val;
        }
        ListNode* t = s->next;
        p->next = NULL;
        r->next = NULL;
        while (true) {
            s->next = r;
            r = s;
            s = t;
            if (t == NULL) {
                break;
            } else {
                t = t->next;
            }
        }
        if (q == NULL) {
            q = r;
        }

        p = head;
        while (p != NULL && q != NULL) {
            if (p->val != q->val) {
                return false;
            }
            p = p->next;
            q = q->next;
        }
        return true;
    }
};
static auto _ = [](){ ios::sync_with_stdio(false); cin.tie(nullptr); return 0; }();