M234.回文链表

linked list, two pointers, https://leetcode.cn/problems/palindrome-linked-list/

请用快慢指针实现 O(1) 空间复杂度。

class Solution {
    ListNode* middleNode(ListNode* head) {
        ListNode* slow = head, *fast = head;
        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        return slow;
    }

    ListNode* reverseList(ListNode* head) {
        ListNode* pre = nullptr, *cur = head;
        while (cur) {
            ListNode* nxt = cur->next;
            cur->next = pre;
            pre = cur;
            cur = nxt;
        }
        return pre;
    }

public:
    bool isPalindrome(ListNode* head) {
        ListNode* mid = middleNode(head);
        ListNode* head2 = reverseList(mid);
        while (head2) {
            if (head->val != head2->val) {
                return false;
            }
            head = head->next;
            head2 = head2->next;
        }
        return true;
    }
};