回文链表 力扣234

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    bool isPalindrome(ListNode* head) {
        ListNode *fast = head;
        ListNode *slow = head;
        ListNode *pre = nullptr;
        ListNode *tmp = nullptr;
        while(fast && fast->next){
            fast = fast->next->next;
            tmp = slow->next;
            slow->next = pre;
            pre = slow;
            slow = tmp;
        }
        if(fast != nullptr){
            slow = slow->next;
        }
        while(slow && pre){
            if(slow->val != pre->val){
                return false;
            }
            slow = slow->next;
            pre = pre->next;
        }
        return true;
    }
};

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