<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?
class Solution {
public:
    ListNode* reverse(ListNode* head) {
        ListNode* curr = head;
        ListNode* prev = nullptr;
        ListNode* next = nullptr;

        while (curr != nullptr) {
            next = curr->next;
            curr->next = prev;
            prev = curr;
            curr = next;
        }

        return prev;
    }

    ListNode* getMid(ListNode* head) {
        ListNode* slow = head;
        ListNode* fast = head;

        while (fast != nullptr && fast->next != nullptr) {
            fast = fast->next->next;
            slow = slow->next;
        }

        return slow;
    }

    bool isPalindrome(ListNode* head) {
        if (head == nullptr || head->next == nullptr) {
            return true;
        }

        ListNode* mid = getMid(head);
        ListNode* secondHalf = reverse(mid);
        ListNode* firstHalf = head;
        ListNode* secondHalfCopy = secondHalf;

        while (secondHalf != nullptr) {
            if (firstHalf->val != secondHalf->val) {
                return false;
            }
            firstHalf = firstHalf->next;
            secondHalf = secondHalf->next;
        }

        reverse(secondHalfCopy);

        return true;
    }
};

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