234. Palindrome Linked List（LeetCode）

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) {
       ListNode *p1 = head;
        vector<int> vet;
        if (head == NULL)
            return true;
        else
        {
            while (p1)
            {
                vet.push_back(p1->val);
                p1 = p1->next;
            }
        }
        vector<int> vet1;
        for (int i = vet.size()-1; i >= 0; --i)
        {
            vet1.push_back(vet[i]);
        }       
        if (vet1 == vet)
            return true;
        else
            return false;
    }
};