234. 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) {
        string s;
        while(head != NULL){
            s.push_back(head->val + '0');
            head = head->next;
        }
        string t(s);
        reverse(t.begin(), t.end());
        return s == t ? true : false;
    }
};