day69 234. 回文链表(C++)

题目来源: 234. 回文链表

题目描述：

• 描述: 给你一个单链表的头节点 head ，请你判断该链表是否为回文链表。如果是，返回 true ；否则，返回 false 。
• 示例:

示例1:
输入:head = [1,2,2,1]
输出:true

示例2:
输入:head = [1,2]
输出:false

具体实现:迭代

class Solution {
public:
    bool isPalindrome(ListNode* head) {
        if (head == nullptr) {
            return true;
        }

        // 找到前半部分链表的尾节点并反转后半部分链表
        ListNode* firstHalfEnd = endOfFirstHalf(head);
        ListNode* secondHalfStart = reverseList(firstHalfEnd->next);

        // 判断是否回文
        ListNode* p1 = head;
        ListNode* p2 = secondHalfStart;
        bool result = true;
        while (result && p2 != nullptr) {
            if (p1->val != p2->val) {
                result = false;
            }
            p1 = p1->next;
            p2 = p2->next;
        }        

        // 还原链表并返回结果
        firstHalfEnd->next = reverseList(secondHalfStart);
        return result;
    }

    ListNode* reverseList(ListNode* head) {
        ListNode* prev = nullptr;
        ListNode* curr = head;
        while (curr != nullptr) {
            ListNode* nextTemp = curr->next;
            curr->next = prev;
            prev = curr;
            curr = nextTemp;
        }
        return prev;
    }

    ListNode* endOfFirstHalf(ListNode* head) {
        ListNode* fast = head;
        ListNode* slow = head;
        while (fast->next != nullptr && fast->next->next != nullptr) {
            fast = fast->next->next;
            slow = slow->next;
        }
        return slow;
    }
};