// Definition for singly-linked list.
struct ListNode {
    int val;
    struct ListNode *next;
};

#include <stdbool.h>

struct ListNode* reverse(struct ListNode* node) {
    struct ListNode* prev = NULL;
    while (node) {
        struct ListNode* nextNode = node->next;
        node->next = prev;
        prev = node;
        node = nextNode;
    }
    return prev;
}

bool isPalindrome(struct ListNode* head) {
    if (!head || !head->next) return true;
    
    // Find the middle using fast-slow pointers
    struct ListNode* slow = head;
    struct ListNode* fast = head;
    while (fast->next && fast->next->next) {
        slow = slow->next;
        fast = fast->next->next;
    }
    
    // Reverse the second half
    struct ListNode* secondHalf = reverse(slow->next);
    
    // Compare first half with reversed second half
    struct ListNode* firstHalf = head;
    while (secondHalf) {
        if (firstHalf->val != secondHalf->val) {
            return false;
        }
        firstHalf = firstHalf->next;
        secondHalf = secondHalf->next;
    }
    
    return true;
}