234. Palindrome Linked List

public class Solution {
    public boolean isPalindrome(ListNode head) {
        if(head==null||head.next==null)
        return true;
        ListNode fast=head;
        ListNode low=head;
        ListNode pre=null;
        ListNode cur=head;
        while(fast.next!=null&&fast.next.next!=null){
            ListNode next=cur.next;
            fast=fast.next.next;
            low=low.next;
            cur.next=pre;
            pre=cur;
            cur=next;
        }
        if(fast.next==null){
            cur=cur.next;
        }
        else{ListNode next=cur.next;
        cur.next=pre;
        pre=cur;
        cur=next;
            
        }
        while(cur!=null){
            if(cur.val!=pre.val)
            return false;
            cur=cur.next;
            pre=pre.next;
            
        }
        return true;
    }
}
//更简洁的方法
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isPalindrome(ListNode head) {
        if(head==null||head.next==null)
        return true;
        ListNode fast=head;
        ListNode low=head;
        ListNode pre=null;
        ListNode cur=head;
        while(fast!=null&&fast.next!=null){// you should be careful about that fast!=null must be in front of the statement fast.next!=null
        //Or there may be a nullpointer exception
            ListNode next=cur.next;
            fast=fast.next.next;
            low=low.next;
            cur.next=pre;
            pre=cur;
            cur=next;
        }
        if(fast!=null){ //you cannot use fast.next==null Or there may be a null pointer exception. 
        //Actually if you use while(a!=null&&a.next!=null) in the following you cannot use a.next. you can only use a, Or there may be
        a null pointer exception.
            cur=cur.next;
        }
      
        while(cur!=null){
            if(cur.val!=pre.val)
            return false;
            cur=cur.next;
            pre=pre.next;
            
        }
        return true;
    }
}