[LeetCode]234. Palindrome Linked List

https://leetcode.com/problems/palindrome-linked-list/

對input進行修改，後半段反轉。但是反轉連結清單還是不熟啊，這怎麼行！！！反轉之後的尾節點的next為null。剩下看注釋

/**
 * 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 fast = head;
        ListNode slow = head;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }
        // 總結點數為奇數，為滿足下方while判斷，slow要後移一位
        if (fast != null) {
            slow = slow.next;
        }
        ListNode tail = reverse(slow);
        // 不能是head != null，因為前半段的尾節點未置為空
        while (tail != null) {
            if (tail.val != head.val) {
                return false;
            }
            tail = tail.next;
            head = head.next;
        }
        return true;
    }
    private ListNode reverse(ListNode head) {
        ListNode pre = null;
        while (head != null) {
            ListNode next = head.next;
            head.next = pre;
            pre = head;
            head = next;
        }
        return pre;
    }
}