/*
 * @lc app=leetcode id=234 lang=java
 *
 * [234] Palindrome Linked List
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    /**
     * linked list求中点方案：快慢指针：
     * 当fast移动n次时，列表节点个数为1+2n或2n
     * 此时，slow移动n次时遍历节点个数为1+n
     * 即，节点总个数为奇数时，slow刚好位于中点
     * 当节点总个数为偶数时，slow位于n+1节点，即下半部分链表的开始节点
     * @param head
     * @return
     */
    public boolean isPalindrome(ListNode head) {
        ListNode slow = head, fast = head, prev = null, t;
        while(fast != null && fast.next != null) {
            fast = fast.next.next;
            t = slow.next;
            slow.next = prev;
            prev = slow;
            slow = t;
        }
        // 此时，prev为上半部分开始节点，cur为下半部分开始节点
        // 如果fast不为空，节点总数为奇数，fast为空，节点总数为偶数
        if(fast != null) {
            slow = slow.next;
        }
        boolean f = true;
        while(slow != null) {
            if(slow.val != prev.val) {
                f = false;
            }
            slow = slow.next;
            prev = prev.next;
        }
        return f;
    }
}
// @lc code=end