234. 回文链表

Posted 2021-10-05 潜行前行

234. 回文链表
     给你一个单链表的头节点 head ，请你判断该链表是否为回文链表。如果是，返回 true ；否则，返回 false 。

示例 1：
输入：head = [1,2,2,1]
输出：true
示例 2：
输入：head = [1,2]
输出：false

提示：
链表中节点数目在范围[1, 105] 内
0 <= Node.val <= 9

进阶：你能否用 O(n) 时间复杂度和 O(1) 空间复杂度解决此题？

题解

/**
 * 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 {
    public boolean isPalindrome(ListNode head) {

            ListNode slow = head;
            ListNode reverse = null;
            if (head.next != null) {
                ListNode fast = head.next, tmp;
                while (fast.next!=null && fast.next.next != null) {
                    fast = fast.next.next;
                    tmp = reverse;
                    reverse = slow;
                    slow = slow.next;
                    reverse.next = tmp;
                }
                tmp = reverse;
                reverse = slow;
                slow = fast.next != null ? slow.next.next :slow.next;
                reverse.next = tmp;

                while (slow != null) {
                    if (reverse.val != slow.val) {
                        return false;
                    }
                    slow = slow.next;
                    reverse = reverse.next;
                }
            }
            return true;
        }
}