0234.cpp

/**
 * 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 || !head->next) return true;

        auto slow = head;
        auto fast = head;
        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
        }

        if (fast) slow = slow->next;
        slow = reverseList(slow);

        while (slow) {
            if (slow->val != head->val) return false;
            slow = slow->next;
            head = head->next;
        }

        return true;
    }

private:
    ListNode* reverseList(ListNode* head) {
        ListNode* prev = NULL;
        ListNode* curr = head;

        while (curr) {
            ListNode* next = curr->next;
            curr->next = prev;
            prev = curr;
            curr = next;
        }

        return prev;
    }
};