234.txt

/**
 * 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;
        ListNode* fast = head, *slow = head;
        while(fast && fast->next){
            slow = slow -> next;
            fast = fast -> next -> next;


        }

        ListNode*  right = revervse(slow);
        ListNode* left = head;
        while(right && left){
            if (right -> val == left -> val){
                right = right -> next;
                left = left -> next;

            }
            else return false;
        }

        return true;

        
        
        
    }
private:
    ListNode* revervse(ListNode* begin){
        ListNode* pre, *cur, *next;
        pre = nullptr;
        cur = begin;
        while(cur){
            next = cur -> next;
            cur -> next = pre;
            pre = cur;
            cur = next;
        }

        return pre;  // pre 返回最后一个结点， cur 返回null；



    }
};