链表回文

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};*/
class Palindrome {
public:
    bool isPalindrome(ListNode* pHead)
    {
        // write code here
          if(pHead == NULL)
            return true;
        stack<int> ss;
        ListNode* p = pHead;
        ListNode* q = pHead;
        ss.push(p->val);
        while(q->next != NULL && q->next->next != NULL)
            {
            p = p->next;
           ss.push(p->val);
            q = q->next->next;
        }
        if(q->next == NULL)                                                    //长度为奇数
            ss.pop();
        p = p->next;
        while(!ss.empty())
            {
            if(ss.top() != p->val)
                break;
            p = p->next;
            ss.pop();
        }
        if(ss.empty())
            return true;
        else
            return false;
    }     

};

class Palindrome {
public:
    bool isPalindrome(ListNode* pHead) {
        ListNode *fast=pHead,*slow=pHead;
        stack<int> stk;
        while(fast!=NULL && fast->next!=NULL)
            {
            stk.push(slow->val);
            slow=slow->next;
            fast=fast->next->next;
        }
        if(fast)  //奇数情况
            slow=slow->next;
        while(slow!=NULL)
            {
              if(stk.top()!=(slow->val))
                return false;
            slow=slow->next;
            stk.pop();
        }
        return true;
    }
};