234. Palindrome Linked List

Posted 2022-08-03 wx62ea2466cca9a

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?

/**
 * 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) 
        ListNode slow=head;  
        ListNode fast=head;  

        if(fast==null||fast.next==null)//0个节点或是1个节点  
             return true;  

        while(fast.next!=null&&fast.next.next!=null)  
          
             fast=fast.next.next;  
             slow=slow.next;  
          
        //对链表后半段进行反转  
        ListNode midNode=slow;  
        ListNode firNode=slow.next;//后半段链表的第一个节点  
        ListNode cur=firNode.next;//插入节点从第一个节点后面一个开始  
        firNode.next=null;//第一个节点最后会变最后一个节点  
        while(cur!=null)  
          
             ListNode nextNode=cur.next;//保存下次遍历的节点  
             cur.next=midNode.next;  
             midNode.next=cur;  
             cur=nextNode;  
          

        //反转之后对前后半段进行比较  
        slow=head;  
        fast=midNode.next;  
        while(fast!=null)  
          
            if(fast.val!=slow.val)  
                return false;  
            slow=slow.next;  
            fast=fast.next;  
          
        return true;