class Solution {
    public boolean isPalindrome(ListNode head) {
        // linear time and constant space
        // Reverse second half, compare. Draw a diagram if not clear(odd 3, even 4)
        // 99% 60%
        if (head == null || head.next == null) return true;
        
        ListNode slow = head, fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        } // slow should be in the middle or begin of second half;
        
        
        slow = reverse(slow);
        
        while(head != null && slow != null) {
            if (head.val != slow.val) return false;
            head = head.next;
            slow = slow.next;
        }
        return true;
        
    }
    
    private ListNode reverse(ListNode head) {
        ListNode pre = null, curr = head;
        while (curr != null) {
            ListNode next = curr.next;
            curr.next = pre;
            pre = curr;
            curr = next;
        }
        return pre;
    }
}