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?

/**

 * Definition for singly-linked list.

 * struct ListNode {

 *     int val;

 *     ListNode *next;

 *     ListNode(int x) : val(x), next(NULL) {}

 * };

 */

 /*

 求长度，找中点，从中点

 */

class Solution {

public:

    ListNode* reverseList(ListNode* head)

    {

        ListNode *p=head,*newHead = NULL,*next=NULL;

        while(p){

            next = p->next;

            p->next = newHead;

            newHead = p;

            p = next;

        }

        return newHead;

    }

    bool isPalindrome(ListNode* head) {

        if(head==NULL || head->next==NULL){

            return true;

        }

        int len = ;

        ListNode* p = head;

        while(p){

            len++;

            p=p->next;

        }

        int m = len/;

        p = head;

        while(--m){

            p=p->next;

        }

        ListNode *node = p;

        if(len%){

            p = p->next->next;

            ListNode *tmp = node->next;

            node->next = NULL;

            delete(tmp);

        }else{

            p = p->next;

            node->next = NULL;

        }

        ListNode *newHead = reverseList(head);

        while(newHead && p){

            if(newHead->val == p->val){

                newHead=newHead->next;

                p = p->next;

            }else{

                return false;

            }

        }

        return true;

       // while()

    }

};