Given a singly linked list, determine if it is a palindrome.

Follow up:
Could you do it in O(n) time and O(1) space?

bool isPalindrome(ListNode* head) {
    if (head == NULL || head->next == NULL)
        return true;

    ListNode *fast = head;
    ListNode *slow = head;
    while (fast->next != NULL)
    {
        fast = fast->next;
        slow = slow->next;
        if (fast->next != NULL)
            fast = fast->next;
    }
    fast = reverseList(slow);
    ListNode *p = head;
    while (p != slow && p->val == fast->val)
    {
        p = p->next;
        fast = fast->next;
    }
    if (p == slow)
        return true;
    else
        return false;
}