Leetcode 234 回文联表(Palindrome Linked List) 题解分析

Given a singly linked list, determine if it is a palindrome.

Example 1:

Input: 1->2
Output: false
Example 2:

Input: 1->2->2->1
Output: true
Follow up:
Could you do it in O(n) time and O(1) space?

/**
 * 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) {
        if (head == null) {
            return true;
        }
        ListNode tail = head;
        LinkedList<Integer> stack = new LinkedList<>();
        // 循环入栈
        while (tail != null) {
            stack.push(tail.val);
            tail = tail.next;
        }
        // 再遍历链表跟出栈的对比，其实就是正反序的对比
        while (!stack.isEmpty()) {
            if (stack.peekFirst() == head.val) {
                stack.pollFirst();
                head = head.next;
            } else {
                return false;
            }
        }
        return true;

//        return stack.isEmpty();
    }
}