【LeetCode】234.回文链表

题目

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

解答

源代码

/**
 * 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 halfList = getHalfList(head);
        System.out.println(halfList.val);
        ListNode reversedList = reverse(halfList);
        System.out.println(reversedList.val);

        while (reversedList != null) {
            if (head.val != reversedList.val) {
                return false;
            }

            reversedList = reversedList.next;
            head = head.next;
        }

        return true;
    }

    public ListNode getHalfList(ListNode head) {
        ListNode fast = head, slow = head;

        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }

        return slow;
    }

    public ListNode reverse(ListNode head) {
        ListNode pre = null;
        ListNode cur = head;

        while (cur != null) {
            ListNode next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }

        return pre;
    }
}