java 回文链表四

 回文链表

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) {
        // 使用快慢指针
        ListNode fast = head;
        ListNode slow = head;
        // 快指针每次都比慢指针多走一步，当快指针到尾节点，刚好慢指针走到中间
        while(fast!=null&&fast.next!=null)
        {
               fast = fast.next.next;
               slow = slow.next;
        }
        //  从中开始的慢指针进行逆置，
        ListNode cur = slow,prv = null,temp=null;
        while(cur!=null)
        {
            temp = cur.next;
            cur.next = prv;
            prv = cur;
            cur = temp;
        }
        while(prv!=null)
        {
            if(prv.val!=head.val)
                return false;
            prv = prv.next;
            head =head.next;
        }
        return true;
    }
}