Palindrome Linked List

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

Example 1:

Input: 1->2
Output: false

Example 2:

Input: 1->2->2->1
Output: true

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

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isPalindrome(ListNode head) {
        //find mid point
        
        if(head == null ) 
            return true;
       
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        ListNode slow = dummy,fast = dummy;
        
        while(fast != null && fast.next != null){
            slow = slow.next;
            fast = fast.next.next;
        }
        
        
        
        //now slow is on mid point;
        
         //reverse second half
        reverse(slow);
        //compare first half to second half
        ListNode runner1 = dummy.next;
        ListNode runner2 = slow.next;
        
        while(runner2 != null){
            if(runner2.val != runner1.val){
                return false;
            }
            runner2 = runner2.next;
            runner1 = runner1.next;
        }
        
        return true;
        
        // dummy -> 1 -> 2 -> 3 -> 4 -> 5  -> 4 -> 3 -> 2 -> 1
    }
    
    public void reverse(ListNode head){
        //this head is the one node before the list need to reverse
        if(head == null || head.next == null) return;
        
        ListNode cur = head.next;
        ListNode pre = null, next = null;
        
        while(cur != null){
            next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        
        head.next = pre;
        
    }
}