LeetCode:Palindrome Linked List

Palindrome Linked List
Total Accepted: 28267 
Total Submissions: 113712 
Difficulty: Easy
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?
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考え方:
1.「速い」ポインタと「遅い」ポインタで、同時に後ろに移動します.2つのポインタの後方移動速度は、fast=2*slow、すなわちslowが1歩、fastが2歩です.
2.slowポインタの最終位置チェーンテーブルの中間位置.
3.前半のチェーンテーブルを逆さにします.
4.2つの部分を比較します.
code:

/**
 * 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) {
            return true;
        }
        ListNode slow = head;
        ListNode fast = head;
        ListNode p = slow.next;
        ListNode pre = slow;
        //find mid pointer, and reverse head half part
        while(fast.next != null &&fast.next.next != null) {
            fast = fast.next.next;
            pre = slow;
            slow = p;
            p = p.next;
            slow.next = pre;
        }

        //odd number of elements, need left move slow one step
        if(fast.next == null) slow = slow.next;
            slow = slow.next;
            p = p.next;
        }
        return true;

    }
}