leetcode---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?

解题思路是：
1、找到链表的中间位置
2、把后半部分进行反转
3、逐位比较前半部分与后半部分是不是相等。

/** * 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;
        //只有两个值的链，需要单独判断
        if(head.next.next==null) return head.val==head.next.val;
        //快慢指针找到中间的节点
        ListNode fast=head,slow=head;
        while(fast.next!=null&&fast.next.next!=null){
            fast=fast.next.next;
            slow=slow.next;
        }

        ListNode mid=reserve(slow.next);
        ListNode curr=head;
        while(mid!=null){
            if(curr.val!=mid.val) 
                 return false;
            curr=curr.next;
            mid=mid.next;
        }
         return true;  
    }

    //反转单链表
    public ListNode reserve(ListNode head){
        if(head==null) return head;
        ListNode curr=head,pre=null,tmp=null;
        while(curr!=null){
            tmp=curr.next;
            curr.next=pre;
            pre=curr;
            curr=tmp;
        }
       return pre; 
    }

}