leetcode：Palindrome Linked List 【Java】

一、问题描述

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?
二、问题分析

把后半部分链表翻转（参照博文leetcode：Reverse
Linked List 【Java】）后，与前半部分比较。

三、算法代码

/**
 * 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;
        }
        //1.遍历确定长度
        int length = 0;
        ListNode p = head;
        while(p != null) {
            length ++;
            p = p.next;
        }
        p = head;//用完之后, p归位
        if(length == 1) {
            return true;
        }
        //2.将后半部分链反转
        int half = (length + 1) / 2;
        ListNode q = head;
        for(int i = 0; i < half; i ++) {
            q = q.next;
        }
        //开始反转
        ListNode r = q.next;
        q.next = null;
        ListNode m;
        while(r != null) {
            m = r.next;
            r.next = q;
            q = r;
            r = m;
        }
        //3.依次比较,直到其中一个或者两个链遍历完
        while(q != null && p != null) {
            if(p.val == q.val) {
                q = q.next;
                p = p.next;
            }else {
                return false;
            }
        }
        return true;
    }
}