<p>Given the <code>head</code> of a singly linked list, return <code>true</code><em> if it is a </em><span data-keyword="palindrome-sequence"><em>palindrome</em></span><em> or </em><code>false</code><em> otherwise</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/03/03/pal1linked-list.jpg" style="width: 422px; height: 62px;" />
<pre>
<strong>Input:</strong> head = [1,2,2,1]
<strong>Output:</strong> true
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/03/03/pal2linked-list.jpg" style="width: 182px; height: 62px;" />
<pre>
<strong>Input:</strong> head = [1,2]
<strong>Output:</strong> false
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the list is in the range <code>[1, 10<sup>5</sup>]</code>.</li>
	<li><code>0 &lt;= Node.val &lt;= 9</code></li>
</ul>

<p>&nbsp;</p>
<strong>Follow up:</strong> Could you do it in <code>O(n)</code> time and <code>O(1)</code> space?
/**
 * 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) {}
 * };
 */
ListNode *findMid(ListNode *head){
    if(head==NULL) return head;

    ListNode *slow=head;
    ListNode *fast=head;
    while(fast!=NULL && fast->next!=NULL){
        fast=fast->next->next;
        slow=slow->next;
    }
    return slow;
}
ListNode *reverse(ListNode *head){
    if(head==NULL || head->next==NULL) return head;
    ListNode *temp=NULL;
    ListNode *cur=head;
    ListNode *pre=NULL;
    while(cur!=NULL){
        temp=cur->next;
        cur->next=pre;
        pre=cur;
        cur=temp;
    }
    return pre;
}
class Solution {
public:
    bool isPalindrome(ListNode* head) {
        ListNode *mid=findMid(head);
        mid=reverse(mid);
        while(mid!=NULL){
            if(head->val!=mid->val){
                return false;
            }
            head=head->next;
            mid=mid->next;
        }
        return true;
    }
};
48645