链表的回文结构

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};*/
class PalindromeList {
public:
    bool chkPalindrome(ListNode* A) {
        // write code here
        if(A == NULL) return false;
        
        bool flag = true;
        ListNode *firstNode = A;
        ListNode *secondNode = A;
        
        while(secondNode->next != NULL) {
            secondNode = secondNode->next;
        }
        
        while(firstNode < secondNode) {
            if(firstNode->val != secondNode->val) {
                flag = false;
                break;
            }
            firstNode = firstNode->next;
            secondNode = updateSecondNode(A, secondNode);
        }
        return flag;
    }
    
    ListNode* updateSecondNode(ListNode *A, ListNode *secondNode) {
        ListNode *preNode = A;
        while(preNode->next != secondNode) {
               preNode = preNode->next;
        }
        return preNode;       
    }
};