LeetCode 0234 - Palindrome Linked List

Input: 1->2
Output: false

Input: 1->2->2->1
Output: true

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
private:
    ListNode* tmp = nullptr;
    bool check(ListNode* p) {
        if(nullptr == p) {
            return true;
        }
        bool flag = check(p->next) && (p->val == tmp->val);
        tmp = tmp->next;
        return flag;
    }
public:
    bool isPalindrome(ListNode* head) {
        tmp = head;
        return check(head);
    }
};