234.回文链表

/**
 * 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==nullptr || head->next==nullptr)  //注意初始判定条件
            return true;
        ListNode *pre=nullptr,*prepre=nullptr,*fast=head->next,*slow=head;  //pre，prepre要初始化为空指针
        while(fast!=nullptr && fast->next!=nullptr){
            pre = slow;
            slow = slow->next;
            fast = fast->next->next;
            pre->next = prepre;  //翻转指针
            prepre = pre;    
        }
        ListNode *p2 = slow->next;
        slow->next = pre;
        ListNode *p1 = fast == nullptr?slow->next:slow;   //列表size奇数与偶数对应不同操作
        while(p1!=nullptr){
            if(p1->val!=p2->val)
                return false;
            p1 = p1->next;
            p2 = p2->next;
        }
        return true;
    }
};