题解 | 链表的回文结构

import java.util.*;

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class PalindromeList {
    public boolean chkPalindrome(ListNode head) {
        // write code here
        ListNode fast = head;
        ListNode slow = head;
        ListNode headA = head;

        // 找到中间节点
        while(fast != null && fast.next != null){
            fast = fast.next.next;
            slow = slow.next;
        }


        // 开始反转中间节点之后
        // cur 表示当前要反转的节点
        // curN 记录cur后面的节点，避免反转后丢失
        // slow 表示反转后cur节点的下一个节点
        ListNode cur = slow.next;
        while(cur != null){
            ListNode curN = cur.next;
            cur.next = slow;
            slow = cur;
            cur = curN;
        }


        //两头分别向后走，判断是不是回文结构,奇数直到相遇，偶数直到headA.next = slow;
        while(headA != slow){
            if(headA.val != slow.val){
                return false;
            }

            if(headA.next == slow){
                return true;
            }
            headA = headA.next;
            slow = slow.next;
        }
        return true;

        
    }
}