234. Palindrome Linked List

Given a singly linked list, determine if it is a palindrome.

Follow up:
Could you do it in O(n) time and O(1) space?

先用2 pointer查找到中间点，然后后半部分reverse， 最后从head与中间点开始挨个check。此处要注意奇偶。

public bool IsPalindrome(ListNode head) {
         if(head == null) return true;
         ListNode slow = head;
         ListNode fast = head;
         ListNode nextNode = null;
         ListNode newHead = null;
         
         //get middle pointer of the linked list
         while(fast != null && fast.next != null)
         {
             slow = slow.next;
             fast = fast.next.next;
         }
         
          while(slow != null)
             {
                 nextNode = slow.next;
                 slow.next = newHead;
                 newHead = slow;
                 slow = nextNode;
             }
         while(head != null && newHead != null)
         {
             if(head.val != newHead.val) return false;
             head = head.next;
             newHead = newHead.next;
         }
         return true;
     }