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?

判断单链表是否是回文的

C++（25ms）：

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