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?

 

Analyse: Obtain the elements first, then judge. 

Runtime: 28ms.

/**
 * 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) {
        if(!head || !head->next) return true;
        
        vector<int> v;
        while(head){
            v.push_back(head->val);
            head = head->next;
        }
        for(int i = 0, j = v.size() - 1; i < j; i++, j--){
            if(v[i] != v[j]) return false;
        }
        return true;
    }
};