<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) {}
 * };
 */
class Solution {
public:
    ListNode* fr(ListNode* h){
        ListNode *p=NULL,*q=h,*r;
        while(q){
            r=q->next;
            q->next=p;
            p=q;
            q=r;
        }
        return p;
    }
    bool isPalindrome(ListNode* head) {
        ListNode *s=head,*f=head;
        int y=0;
        ListNode* q=head;
        while(q){
            q=q->next;y++;
        }
        while(f->next!=NULL && f->next->next!=NULL){
            s=s->next;
            f=f->next->next;
        }
        f=s->next;
        q=f;
        s->next=NULL;
        s=head;
        s=fr(s);
        if(y&1){
           s=s->next; 
        }
        while(s && f){
            if((s->val)!=(f->val)){
                return 0;
            }
            s=s->next;
            f=f->next;
        }
        s=head;
        s=fr(s);
        while(s->next){
            s=s->next;
        }
        s->next=q;
        return 1;
    }
};
24795