Palindrome Linked List

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) {}
 * };
 */
class Solution {
public:
    bool isPalindrome(ListNode* head) {
        
        vector<int> tmp;
        while(head)
        {
            tmp.push_back(head->val);
            head=head->next;
        }
        int left=0;
        int right=tmp.size()-1;
        while(left<right)
        {
            if(tmp[left]!=tmp[right])
              return false;
            left++;
            right--;
        }
        return true;
    }
};