234. Palindrome Linked List

Follow up:

Could you do it in O(n) time and O(1) space?
/**
 * 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)
            return true;
        ListNode * slow = head;
        ListNode * fast = head;
        ListNode * mid = NULL;
        while(fast && fast->next){
            slow = slow->next;
            fast = fast->next->next;
        }
        
        if(fast){
            //奇数个元素
            slow->next = reverseList(slow->next);
            slow = slow->next;
        }else{
            //偶数个元素
            slow = reverseList(slow);
        }
        
        while(slow){
            if(slow->val!=head->val)
            return false;
            else{
                slow = slow->next;
                head = head->next;
            }
        }
        return true;
    }
    
    ListNode* reverseList(ListNode* head) {
        if(!head)
            return head;
        ListNode * Dummy = new ListNode(-1);
        ListNode * pCur = head;
        ListNode * pNext = nullptr;
        while(pCur){
            pNext = pCur->next;
            pCur->next = Dummy->next;
            Dummy->next = pCur;
            pCur = pNext;
        }
        return Dummy->next;
        delete Dummy;
    }
};